Table of Contents
End of Product Lifecycle. Active development and support for MySQL Database Server versions 3.23, 4.0, and 4.1 has ended. For details, see http://www.mysql.com/about/legal/lifecycle/#calendar. Please consider upgrading to a recent version. Further updates to the content of this manual will be minimal. All formats of this manual will continue to be available until 31 Dec 2010.
MySQL Server (mysqld) is the main program that does most of the work in a MySQL installation. This section provides an overview of MySQL Server and covers topics that deal with administering a MySQL installation:
Server configuration
The server log files
Security issues and user-account management
Management of multiple servers on a single machine
mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:
Startup options that the server supports
Server system variables
Server status variables
How to set the server SQL mode
The server shutdown process
Not all storage engines (also known in older versions of MySQL as “table types”) are supported by all MySQL server binaries and configurations. To find out how to determine which storage engines your MySQL server installation supports, see Section 12.4.5.10, “SHOW ENGINES Syntax”.
The following table provides a list of all the command line
options, server and status variables applicable within
mysqld.
For a version of this table that is specific to MySQL Cluster, see Section 15.3.4.1, “MySQL Cluster Server Option and Variable Reference”.
The table lists command-line options (Cmd-line), options valid in configuration files (Option file), server system variables (System Var), and status variables (Status var) in one unified list, with notification of where each option/variable is valid. If a server option set on the command line or in an option file differs from the name of the corresponding server system or status variable, the variable name is noted immediately below the corresponding option. For status variables, the scope of the variable is shown (Scope) as either global, session, or both. Please see the corresponding sections for details on setting and using the options and variables. Where appropriate, a direct link to further information on the item as available.
Table 5.1 Option/Variable Summary
When you start the mysqld server, you can specify program options using any of the methods described in Section 4.2.3, “Specifying Program Options”. The most common methods are to provide options in an option file or on the command line. However, in most cases it is desirable to make sure that the server uses the same options each time it runs. The best way to ensure this is to list them in an option file. See Section 4.2.3.3, “Using Option Files”.
mysqld reads options from the
[mysqld] and [server]
groups. mysqld_safe reads options from the
[mysqld], [server],
[mysqld_safe], and
[safe_mysqld] groups.
mysql.server reads options from the
[mysqld] and [mysql.server]
groups.
An embedded MySQL server usually reads options from the
[server], [embedded], and
[
groups, where xxxxx_SERVER]xxxxx is the name of the
application into which the server is embedded.
mysqld accepts many command options. For a
list, execute mysqld --help. Before MySQL
4.1.1, --help prints the full help
message. As of 4.1.1, it prints a brief message; to see the full
list, use mysqld --verbose --help.
The following list shows some of the most common server options. Additional options are described in other sections:
Options that affect security: See Section 5.4.4, “Security-Related mysqld Options”.
SSL-related options: See Section 5.6.6.3, “SSL Command Options”.
Binary log control options: See Section 14.8.4, “Binary Log Options and Variables”.
Replication-related options: See Section 14.8, “Replication and Binary Logging Options and Variables”.
Options specific to particular storage engines: See Section 13.1.1, “MyISAM Startup Options”, Section 13.5.3, “BDB Startup Options”, Section 13.2.4, “InnoDB Startup Options and System Variables”, and Section 15.3.4.2, “mysqld Command Options for MySQL Cluster”.
You can also set the values of server system variables by using variable names as options, as described at the end of this section.
Some options control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to an option that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible that the server will adjust a value upward. For example, if you assign a value of 0 to an option for which the minimal value is 1024, the server will set the value to 1024.
Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.
Some options take file name values. Unless otherwise specified,
the default file location is the data directory if the value is a
relative path name. To specify the location explicitly, use an
absolute path name. Suppose that the data directory is
/var/mysql/data. If a file-valued option is
given as a relative path name, it will be located under
/var/mysql/data. If the value is an absolute
path name, its location is as given by the path name.
--help, -?
Display a short help message and exit. Before MySQL 4.1.1,
--help displays the full help
message. As of 4.1.1, it displays an abbreviated message only.
Use both the --verbose and
--help options to see the full
message.
This option controls whether user-defined functions that have
only an xxx symbol for the main function
can be loaded. By default, the option is off and only UDFs
that have at least one auxiliary symbol can be loaded; this
prevents attempts at loading functions from shared object
files other than those containing legitimate UDFs. This option
was added in MySQL 4.0.24, and 4.1.10a. See
Section 18.2.2.6, “User-Defined Function Security Precautions”.
Use standard (ANSI) SQL syntax instead of MySQL syntax. For
more precise control over the server SQL mode, use the
--sql-mode option instead. See
Section 1.9.3, “Running MySQL in ANSI Mode”, and
Section 5.1.6, “Server SQL Modes”.
The path to the MySQL installation directory. All paths are usually resolved relative to this directory.
Enable large result sets by saving all temporary sets in files. This option prevents most “table full” errors, but also slows down queries for which in-memory tables would suffice. Since MySQL 3.23.2, the server is able to handle large result sets automatically by using memory for small temporary tables and switching to disk tables where necessary.
The IP address to bind to. Only one address can be selected. If this option is specified multiple times, the last address given is used.
If no address or 0.0.0.0 is specified, the
server listens on all interfaces.
This option is used by the mysql_install_db script to create the MySQL privilege tables without having to start a full MySQL server.
The directory where character sets are installed. See Section 9.6, “Character Set Configuration”.
--character-set-client-handshake
Do not ignore character set information sent by the client. To
ignore client information and use the default server character
set, use
--skip-character-set-client-handshake;
this makes MySQL 4.1 and higher behave like MySQL 4.0. This
option was added in MySQL 4.1.15.
--character-set-server=,
charset_name-C
charset_name
Use charset_name as the default
server character set. See
Section 9.6, “Character Set Configuration”. If you use this
option to specify a nondefault character set, you should also
use --collation-server to
specify the collation. This option is available as of MySQL
4.1.3.
--chroot=,
path-r
path
Put the mysqld server in a closed
environment during startup by using the
chroot() system call. This is a recommended
security measure as of MySQL 4.0. (MySQL 3.23 is not able to
provide a chroot() jail that is 100%
closed.) Note that use of this option somewhat limits
LOAD DATA
INFILE and
SELECT ... INTO
OUTFILE.
--collation-server=
collation_name
Use collation_name as the default
server collation. This option is available as of MySQL 4.1.3.
See Section 9.6, “Character Set Configuration”.
(Windows only.) Write error log messages to
stderr and stdout even
if --log-error is specified.
mysqld does not close the console window if
this option is used.
Write a core file if mysqld dies. The name
and location of the core file is system dependent. On Linux, a
core file named
core. is
written to the current working directory of the process, which
for mysqld is the data directory.
pidpid represents the process ID of
the server process. On Mac OS X, a core file named
core. is
written to the pid/cores directory. On
Solaris, use the coreadm command to specify
where to write the core file and how to name it.
For some systems, to get a core file you must also specify the
--core-file-size option to
mysqld_safe. See
Section 4.3.2, “mysqld_safe — MySQL Server Startup Script”. On some systems, such as
Solaris, you do not get a core file if you are also using the
--user option. There might be
additional restrictions or limitations. For example, it might
be necessary to execute ulimit -c unlimited
before starting the server. Consult your system documentation.
--datadir=,
path-h
path
The path to the data directory.
--debug[=,
debug_options]-# [
debug_options]
If MySQL is configured with
--with-debug, you can use
this option to get a trace file of what
mysqld is doing. A typical
debug_options string is
'd:t:o,.
The default is file_name''d:t:i:o,mysqld.trace'.
--default-character-set=,
charset_name-C
charset_name
Use charset_name as the default
character set. This option is deprecated in favor of
--character-set-server as of
MySQL 4.1.3. See Section 9.6, “Character Set Configuration”.
For more information, see Section 18.4.3, “The DBUG Package”.
--default-collation=
collation_name
Use collation_name as the default
collation. This option is deprecated in favor of
--collation-server as of MySQL
4.1.3. See Section 9.6, “Character Set Configuration”.
This option is a synonym for
--default-table-type. It is
available as of MySQL 4.1.2.
Set the default table type (storage engine) for tables. See Chapter 13, Storage Engines.
Set the default server time zone. This option sets the global
time_zone system variable. If
this option is not given, the default time zone is the same as
the system time zone (given by the value of the
system_time_zone system
variable. This option is available as of MySQL 4.1.3.
--delay-key-write[={OFF|ON|ALL}]
Specify how to use delayed key writes. Delayed key writing
causes key buffers not to be flushed between writes for
MyISAM tables. OFF
disables delayed key writes. ON enables
delayed key writes for those tables that were created with the
DELAY_KEY_WRITE option.
ALL delays key writes for all
MyISAM tables. Available as of MySQL 4.0.3.
See Section 7.8.2, “Tuning Server Parameters”, and
Section 13.1.1, “MyISAM Startup Options”.
If you set this variable to ALL, you
should not use MyISAM tables from within
another program (such as another MySQL server or
myisamchk) when the tables are in use.
Doing so leads to index corruption.
--delay-key-write-for-all-tables
Old form of
--delay-key-write=ALL for use
prior to MySQL 4.0.3. As of 4.0.3, use
--delay-key-write=ALL instead.
--delay-key-write-for-all-tables
is removed in MySQL 5.5.
Read the default DES keys from this file. These keys are used
by the DES_ENCRYPT() and
DES_DECRYPT() functions.
This option is deprecated. Use
--external-locking instead.
Enable support for named pipes. This option applies only on Windows NT, 2000, XP, and 2003 systems, and can be used only with the mysqld-nt and mysqld-max-nt servers that support named-pipe connections.
Print a symbolic stack trace on failure. This capability is
available only on Intel Linux systems, and only if MySQL was
configured with the --with-pstack option.
--exit-info[=,
flags]-T [
flags]
This is a bit mask of different flags that you can use for debugging the mysqld server. Do not use this option unless you know exactly what it does!
Enable external locking (system locking), which is disabled by
default as of MySQL 4.0. Note that if you use this option on a
system on which lockd does not fully work
(such as Linux), it is easy for mysqld to
deadlock. This option was named
--enable-locking before MySQL 4.0.3.
For more information about external locking, including conditions under which it can and cannot be used, see Section 7.6.4, “External Locking”.
Flush (synchronize) all changes to disk after each SQL statement. Normally, MySQL does a write of all changes to disk only after each SQL statement and lets the operating system handle the synchronizing to disk. See Section B.5.4.2, “What to Do If MySQL Keeps Crashing”.
Install an interrupt handler for SIGINT
(needed to stop mysqld with
^C to set breakpoints) and disable stack
tracing and core file handling. See Section 18.4, “Porting to Other Systems”.
This option was added in MySQL 4.0.14.
Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.
If this option is given, then after a crash recovery by
InnoDB, mysqld truncates
the binary log after the last not-rolled-back transaction in
the log. The option also causes InnoDB to
print an error if the binary log is smaller or shorter than it
should be. See Section 5.3.4, “The Binary Log”.
--innodb-
xxx
Set an option for the InnoDB storage
engine. The InnoDB options are listed in
Section 13.2.4, “InnoDB Startup Options and System Variables”.
| Command-Line Format | --install [service_name] | ||
| Platform Specific | Windows | ||
(Windows only) Install the server as a Windows service that
starts automatically during Windows startup. The default
service name is MySQL if no
service_name value is given. For
more information, see Section 2.3.11, “Starting MySQL as a Windows Service”.
--install-manual
[
service_name]
| Command-Line Format | --install-manual [service_name] | ||
| Platform Specific | Windows | ||
(Windows only) Install the server as a Windows service that
must be started manually. It does not start automatically
during Windows startup. The default service name is
MySQL if no
service_name value is given. For
more information, see Section 2.3.11, “Starting MySQL as a Windows Service”.
--language=
lang_name,
-L lang_name
The language to use for error messages.
lang_name can be given as the
language name or as the full path name to the directory where
the language files are installed. See
Section 9.3, “Setting the Error Message Language”.
--log[=,
file_name]-l [
file_name]
Log connections and SQL statements received from clients to
this file. See Section 5.3.2, “The General Query Log”. If you omit the
file name, MySQL uses
host_name.log
Log errors and startup messages to this file. See
Section 5.3.1, “The Error Log”. If you omit the file name, MySQL
uses
host_name.err.err.
Log all ISAM/MyISAM
changes to this file (used only when debugging
ISAM/MyISAM).
Log extra information to the update log, binary update log,
and slow query log, if they have been activated. For example,
the user name and timestamp are logged for queries. Before
MySQL 4.1, if you are using
--log-slow-queries and
--log-long-format, queries that
are not using indexes also are logged to the slow query log.
--log-long-format is deprecated
as of MySQL version 4.1, when
--log-short-format was
introduced. (Long log format is the default setting since
version 4.1.) Also note that starting with MySQL 4.1, the
--log-queries-not-using-indexes
option is available for the purpose of logging queries that do
not use indexes to the slow query log.
--log-queries-not-using-indexes
If you are using this option with the slow query log enabled, queries that are expected to retrieve all rows are logged. See Section 5.3.5, “The Slow Query Log”. This option does not necessarily mean that no index is used. For example, a query that uses a full index scan uses an index but would be logged because the index would not limit the number of rows. This option is available as of MySQL 4.1.
Originally intended to log less information to the update log, binary log and slow query log, if they have been activated. This option was introduced in MySQL 4.1, but is not operational.
Log slow administrative statements such as
OPTIMIZE TABLE,
ANALYZE TABLE, and
ALTER TABLE to the slow query
log.
This option was added in MySQL 4.1.13. (It is unnecessary in MySQL 4.0 because slow administrative statements are logged by default.)
--log-slow-queries[=
file_name]
Log all queries that have taken more than
long_query_time seconds to
execute to this file. See Section 5.3.5, “The Slow Query Log”.
Note that the default for the amount of information logged has
changed in MySQL 4.1. See the
--log-long-format and
--log-short-format options for
details.
Log updates to fileN where
N is a unique number if not given.
See Section 5.3.3, “The Update Log”. The update log is now
deprecated; you should use the binary log instead
(--log-bin). See
Section 5.3.4, “The Binary Log”.
--log-warnings[=,
level]-W [
level]
Print out warnings such as Aborted
connection... to the error log. Enabling this option
by setting it greater than 0 is recommended, for example, if
you use replication (you get more information about what is
happening, such as messages about network failures and
reconnections). This option is enabled by default as of MySQL
4.0.19 and 4.1.2; to disable it, use
--log-warnings=0. As of MySQL
4.0.21 and 4.1.3, a level argument
can be given. If omitted, the default
level is 1. If the value is greater
than 1, aborted connections are written to the error log. See
Section B.5.2.11, “Communication Errors and Aborted Connections”.
If a slave server was started with
--log-warnings enabled, the
slave prints messages to the error log to provide information
about its status, such as the binary log and relay log
coordinates where it starts its job, when it is switching to
another relay log, when it reconnects after a disconnect, and
so forth.
Before MySQL 4.0.21 and 4.1.3, this is a boolean option, not
an integer-valued option. Before 4.0, this option was named
--warnings.
Give table-modifying operations
(INSERT,
REPLACE,
DELETE,
UPDATE) lower priority than
selects. This can also be done using {INSERT |
REPLACE | DELETE | UPDATE} LOW_PRIORITY ... to lower
the priority of only one query, or by SET
LOW_PRIORITY_UPDATES=1 to change the priority in one
thread. This affects only storage engines that use only
table-level locking (MyISAM,
MEMORY, MERGE). See
Section 7.6.2, “Table Locking Issues”.
Lock the mysqld process in memory. This option might help if you have a problem where the operating system is causing mysqld to swap to disk.
--memlock works on systems that
support the mlockall() system call; this
includes Solaris as well as most Linux distributions that use
a 2.4 or newer kernel. On Linux systems, you can tell whether
or not mlockall() (and thus this option) is
supported by checking to see whether or not it is defined in
the system mman.h file, like this:
shell> grep mlockall /usr/include/sys/mman.h
If mlockall() is supported, you should see
in the output of the previous command something like the
following:
extern int mlockall (int __flags) __THROW;
Using this option requires that you run the server as
root, which, for reasons of security, is
normally not a good idea. See
Section 5.4.6, “How to Run MySQL as a Normal User”.
You must not try to use this option on a system that does
not support the mlockall() system call;
if you do so, mysqld will very likely
crash as soon as you try to start it.
The block size to be used for MyISAM index
pages.
--myisam-recover[=
option[,option]...]]
Set the MyISAM storage engine recovery
mode. The option value is any combination of the values of
DEFAULT, BACKUP,
FORCE, or QUICK. If you
specify multiple values, separate them by commas. You can also
use a value of "" to disable this option.
If this option is used, each time mysqld
opens a MyISAM table, it checks whether the
table is marked as crashed or was not closed properly. (The
last option works only if you are running with external
locking disabled.) If this is the case,
mysqld runs a check on the table. If the
table was corrupted, mysqld attempts to
repair it.
The following options affect how the repair works.
| Option | Description |
|---|---|
DEFAULT | Recovery without backup, forcing, or quick checking. |
BACKUP | If the data file was changed during recovery, save a backup of the
|
FORCE | Run recovery even if we would lose more than one row from the
.MYD file. |
QUICK | do not check the rows in the table if there are not any delete blocks. |
Before the server automatically repairs a table, it writes a
note about the repair to the error log. If you want to be able
to recover from most problems without user intervention, you
should use the options BACKUP,FORCE. This
forces a repair of a table even if some rows would be deleted,
but it keeps the old data file as a backup so that you can
later examine what happened.
See Section 13.1.1, “MyISAM Startup Options”.
This option is available as of MySQL 3.23.25.
The --new option can be used to make the
server behave as 4.1 in certain respects, easing a 4.0 to 4.1
upgrade:
Hexadecimal strings such as 0xFF are
treated as strings by default rather than as numbers.
(Works in 4.0.12 and up.)
TIMESTAMP is returned as a
string with the format 'YYYY-MM-DD
HH:MM:SS'. (Works in 4.0.13 and up.) See
Chapter 10, Data Types.
This option can be used to help you see how your applications behave in MySQL 4.1, without actually upgrading to 4.1.
Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 5.4.2.3, “Password Hashing in MySQL”.
--old-protocol,
-o
Use the 3.20 protocol for compatibility with some very old clients. This option was removed in MySQL 4.1.1.
Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See Section 18.4, “Porting to Other Systems”.
Changes the number of file descriptors available to
mysqld. You should try increasing the value
of this option if mysqld gives you the
error Too many open files.
mysqld uses the option value to reserve
descriptors with setrlimit(). If the
requested number of file descriptors cannot be allocated,
mysqld writes a warning to the error log.
mysqld may attempt to allocate more than
the requested number of descriptors (if they are available),
using the values of
max_connections and
table_cache to estimate
whether more descriptors will be needed.
The path name of the process ID file. The server creates the file in the data directory unless an absolute path name is given to specify a different directory. This file is used by other programs such as mysqld_safe to determine the server's process ID.
--port=,
port_num-P
port_num
The port number to use when listening for TCP/IP connections.
The port number must be 1024 or higher unless the server is
started by the root system user.
| Command-Line Format | --remove [service_name] | ||
| Platform Specific | Windows | ||
(Windows only) Remove a MySQL Windows service. The default
service name is MySQL if no
service_name value is given. For
more information, see Section 2.3.11, “Starting MySQL as a Windows Service”.
Skip some optimization stages.
With this option, the SHOW
DATABASES statement displays only the names of those
databases for which the user has some kind of privilege. As of
MySQL 4.0.2, this option is deprecated and does not do
anything (it is enabled by default), because there is a
SHOW DATABASES privilege that
can be used to control access to database names on a
per-account basis. See Section 5.5.1, “Privileges Provided by MySQL”.
If this option is enabled, a user cannot create new MySQL
users by using the GRANT
statement, if the user does not have the
INSERT privilege for the
mysql.user table or any column in the
table.
Disallow authentication by clients that attempt to use accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
Enable shared-memory connections by local clients. This option is available only on Windows. It was added in MySQL 4.1.0.
--shared-memory-base-name=
name
The name of shared memory to use for shared-memory
connections. This option is available only on Windows. The
default name is MYSQL. The name is case
sensitive. This option was added in MySQL 4.1.0.
Disable the BDB storage engine. This saves
memory and might speed up some operations. Do not use this
option if you require BDB tables.
Turn off the ability to select and insert at the same time on
MyISAM tables. (This is to be used only if
you think you have found a bug in this feature.) See
Section 7.6.3, “Concurrent Inserts”.
Ignore the DELAY_KEY_WRITE option for all
tables. As of MySQL 4.0.3, you should use
--delay-key-write=OFF instead.
See Section 7.8.2, “Tuning Server Parameters”.
Do not use external locking (system locking). For more information about external locking, including conditions under which it can and cannot be used, see Section 7.6.4, “External Locking”.
External locking has been disabled by default since MySQL 4.0.
This option causes the server to start without using the
privilege system at all, which gives anyone with access to the
server unrestricted access to all
databases. You can cause a running server to start
using the grant tables again by executing mysqladmin
flush-privileges or mysqladmin
reload command from a system shell, or by issuing a
MySQL FLUSH
PRIVILEGES statement after connecting to the server.
This option also suppresses loading of user-defined functions
(UDFs).
Do not use the internal host name cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See Section 7.8.5, “How MySQL Uses DNS”.
Disable the InnoDB storage engine. In this
case, the server will not start if the default storage engine
is set to InnoDB. Use
--default-storage-engine to set
the default to some other engine if necessary.
Disable the ISAM storage engine. As of
MySQL 4.1, ISAM is disabled by default, so
this option applies only if the server was configured with
support for ISAM. This option was added in
MySQL 4.1.1.
Disable the MERGE storage engine. This
option was added in MySQL 4.1.21. It can be used if the
following behavior is undesirable: If a user has access to
MyISAM table t,
that user can create a MERGE table
m that accesses
t. However, if the user's
privileges on t are subsequently
revoked, the user can continue to access
t by doing so through
m.
Do not resolve host names when checking client connections.
Use only IP addresses. If you use this option, all
Host column values in the grant tables must
be IP addresses or localhost. See
Section 7.8.5, “How MySQL Uses DNS”.
Do not listen for TCP/IP connections at all. All interaction with mysqld must be made using named pipes or shared memory (on Windows) or Unix socket files (on Unix). This option is highly recommended for systems where only local clients are permitted. See Section 7.8.5, “How MySQL Uses DNS”.
Do not use new, possibly wrong routines.
This is the old form of
--skip-symbolic-links,
for use before MySQL 4.0.13.
--skip-symlink is deprecated as
of 4.0.13 and is removed in MySQL 5.5.
Options that begin with --ssl
specify whether to permit clients to connect using SSL and
indicate where to find SSL keys and certificates. See
Section 5.6.6.3, “SSL Command Options”.
Available on Windows NT-based systems only; instructs the MySQL server not to run as a service.
--symbolic-links,
--skip-symbolic-links
Enable or disable symbolic link support. This option has different effects on Windows and Unix:
On Windows, enabling symbolic links enables you to
establish a symbolic link to a database directory by
creating a
db_name.sym
On Unix, enabling symbolic links means that you can link a
MyISAM index file or data file to
another directory with the INDEX
DIRECTORY or DATA DIRECTORY
options of the CREATE TABLE
statement. If you delete or rename the table, the files
that its symbolic links point to also are deleted or
renamed. See Section 7.10.2, “Using Symbolic Links for Tables on Unix”.
This option was added in MySQL 4.0.13.
If MySQL is configured with
--with-debug=full, all MySQL
programs check for memory overruns during each memory
allocation and memory freeing operation. This checking is very
slow, so for the server you can avoid it when you do not need
it by using the
--skip-safemalloc option.
This option sets the
skip_show_database system
variable that controls who is permitted to use the
SHOW DATABASES statement. See
Section 5.1.3, “Server System Variables”.
do not write stack traces. This option is useful when you are running mysqld under a debugger. On some systems, you also must use this option to get a core file. See Section 18.4, “Porting to Other Systems”.
Disable using thread priorities for faster response time.
mysqld makes a large number of invalid
calls to thread scheduling routines on Linux. These calls do
not affect performance noticeably but may be a source of
“noise” for debugging tools. For example, they
can overwhelm other information of more interest in kernel
logs. To avoid these calls, start the server with the
--skip-thread-priority option.
On Unix, this option specifies the Unix socket file to use
when listening for local connections. The default value is
/tmp/mysql.sock. If this option is given,
the server creates the file in the data directory unless an
absolute path name is given to specify a different directory.
On Windows, the option specifies the pipe name to use when
listening for local connections that use a named pipe. The
default value is MySQL (not case
sensitive).
--sql-mode=
value[,value[,value...]]
Set the SQL mode. See Section 5.1.6, “Server SQL Modes”. This option was added in 3.23.41.
This option causes most temporary files created by the server to use a small set of names, rather than a unique name for each new file. This works around a problem in the Linux kernel dealing with creating many new files with different names. With the old behavior, Linux seems to “leak” memory, because it is being allocated to the directory entry cache rather than to the disk cache.
Sets the default transaction isolation level. The
level value can be
READ-UNCOMMITTED,
READ-COMMITTED,
REPEATABLE-READ, or
SERIALIZABLE. See
Section 12.3.6, “SET TRANSACTION Syntax”.
--tmpdir=,
path-t
path
The path of the directory to use for creating temporary files.
It might be useful if your default /tmp
directory resides on a partition that is too small to hold
temporary tables. Starting from MySQL 4.1.0, this option
accepts several paths that are used in round-robin fashion.
Paths should be separated by colon characters
(“:”) on Unix and semicolon
characters (“;”) on Windows,
NetWare, and OS/2. If the MySQL server is acting as a
replication slave, you should not set
--tmpdir to point to a
directory on a memory-based file system or to a directory that
is cleared when the server host restarts. For more information
about the storage location of temporary files, see
Section B.5.4.4, “Where MySQL Stores Temporary Files”. A replication slave needs
some of its temporary files to survive a machine restart so
that it can replicate temporary tables or
LOAD DATA
INFILE operations. If files in the temporary file
directory are lost when the server restarts, replication
fails.
--user={,
user_name|user_id}-u
{
user_name|user_id}
Run the mysqld server as the user having
the name user_name or the numeric
user ID user_id.
(“User” in this context refers to a system login
account, not a MySQL user listed in the grant tables.)
This option is mandatory when starting
mysqld as root. The
server changes its user ID during its startup sequence,
causing it to run as that particular user rather than as
root. See
Section 5.4.1, “General Security Guidelines”.
Starting from MySQL 3.23.56 and 4.0.12: To avoid a possible
security hole where a user adds a
--user=root option to a
my.cnf file (thus causing the server to
run as root), mysqld
uses only the first --user
option specified and produces a warning if there are multiple
--user options. Options in
/etc/my.cnf and
$MYSQL_HOME/my.cnf are processed before
command-line options, so it is recommended that you put a
--user option in
/etc/my.cnf and specify a value other
than root. The option in
/etc/my.cnf is found before any other
--user options, which ensures
that the server runs as a user other than
root, and that a warning results if any
other --user option is found.
As of MySQL 4.1.1, use this option with the
--help option for detailed
help.
--version, -V
Display version information and exit.
As of MySQL 4.0, you can assign a value to a server system
variable by using an option of the form
--.
For example, var_name=value--key_buffer_size=32M
sets the key_buffer_size variable
to a value of 32MB.
Note that when you assign a value to a variable, MySQL might automatically correct the value to stay within a given range, or adjust the value to the closest permissible value if only certain values are permitted.
If you want to restrict the maximum value to which a variable can
be set at runtime with
SET, you can
define this by using the
--maximum-
command-line option.
var_name=value
It is also possible to set variables by using
--set-variable=
or
var_name=value--
syntax. This syntax is deprecated as of MySQL
4.0.
var_name=value
You can change the values of most system variables for a running
server with the
SET
statement. See Section 12.4.4, “SET Syntax”.
Section 5.1.3, “Server System Variables”, provides a full description for all variables, and additional information for setting them at server startup and runtime. Section 7.8.2, “Tuning Server Parameters”, includes information on optimizing the server by tuning system variables.
The MySQL server maintains many system variables that indicate how
it is configured. Each system variable has a default value. System
variables can be set at server startup using options on the
command line or in an option file. As of MySQL 4.0.3, most of them
can be changed dynamically while the server is running by means of
the SET
statement, which enables you to modify operation of the server
without having to stop and restart it. You can refer to system
variable values in expressions.
There are several ways to see the names and values of system variables:
To see the values that a server will use based on its
compiled-in defaults and any option files that it reads, use
this command (omit --verbose
before MySQL 4.1.1):
mysqld --verbose --help
To see the values that a server will use based on its
compiled-in defaults, ignoring the settings in any option
files, use this command (omit
--verbose before MySQL 4.1.1):
mysqld --no-defaults --verbose --help
For more information, see Section 18.4.3, “The DBUG Package”.
To see the current values used by a running server, use the
SHOW VARIABLES statement.
This section provides a description of each system variable. Variables with no version indicated have been present since at least MySQL 3.22.
The following table lists all available system variables.
Table 5.2 System Variable Summary
| Name | Cmd-Line | Option File | System Var | Var Scope | Dynamic |
|---|---|---|---|---|---|
| autocommit | Yes | Session | Yes | ||
| back_log | Yes | Global | No | ||
| basedir | Yes | Yes | Yes | Global | No |
| bdb_cache_size | Yes | Global | No | ||
| bdb-home | Yes | Yes | No | ||
| - Variable: bdb_home | Yes | Global | No | ||
| bdb-lock-detect | Yes | Yes | Yes | Global | No |
| bdb_log_buffer_size | Yes | Global | No | ||
| bdb-logdir | Yes | Yes | No | ||
| - Variable: bdb_logdir | Yes | Global | No | ||
| bdb_max_lock | Yes | Global | No | ||
| bdb-shared-data | Yes | Yes | No | ||
| - Variable: bdb_shared_data | Yes | Global | No | ||
| bdb-tmpdir | Yes | Yes | No | ||
| - Variable: bdb_tmpdir | Yes | Global | No | ||
| big-tables | Yes | Yes | Yes | ||
| - Variable: big_tables | Yes | Session | Yes | ||
| binlog_cache_size | Yes | Yes | Yes | Global | Yes |
| bulk_insert_buffer_size | Yes | Yes | Yes | Both | Yes |
| character_set | Yes | Yes | Global | No | |
| character_set_client | Yes | Both | Yes | ||
| character_set_connection | Yes | Both | Yes | ||
| character_set_database[a] | Yes | Both | Yes | ||
| character_set_results | Yes | Both | Yes | ||
| character-set-server | Yes | Yes | Yes | ||
| - Variable: character_set_server | Yes | Both | Yes | ||
| character_set_system | Yes | Global | No | ||
| character-sets-dir | Yes | Yes | No | ||
| - Variable: character_sets_dir | Yes | Global | No | ||
| collation_connection | Yes | Both | Yes | ||
| collation_database[b] | Yes | Both | Yes | ||
| collation-server | Yes | Yes | Yes | ||
| - Variable: collation_server | Yes | Both | Yes | ||
| concurrent_insert | Yes | Yes | Yes | Global | Yes |
| connect_timeout | Yes | Yes | Yes | Global | Yes |
| datadir | Yes | Yes | Yes | Global | No |
| date_format | Yes | Both | No | ||
| datetime_format | Yes | Both | No | ||
| debug | Yes | Yes | Yes | Both | Yes |
| default-storage-engine | Yes | Yes | Yes | Both | Yes |
| default_week_format | Yes | Yes | Yes | Both | Yes |
| delay-key-write | Yes | Yes | Yes | ||
| - Variable: delay_key_write | Yes | Global | Yes | ||
| delayed_insert_limit | Yes | Yes | Yes | Global | Yes |
| delayed_insert_timeout | Yes | Yes | Yes | Global | Yes |
| delayed_queue_size | Yes | Yes | Yes | Global | Yes |
| error_count | Yes | Session | No | ||
| expire_logs_days | Yes | Yes | Yes | Global | Yes |
| flush | Yes | Yes | Yes | Global | Yes |
| flush_time | Yes | Yes | Yes | Global | Yes |
| foreign_key_checks | Yes | Session | Yes | ||
| ft_boolean_syntax | Yes | Yes | Yes | Global | Yes |
| ft_max_word_len | Yes | Yes | Yes | Global | No |
| ft_min_word_len | Yes | Yes | Yes | Global | No |
| ft_query_expansion_limit | Yes | Yes | Yes | Global | No |
| ft_stopword_file | Yes | Yes | Yes | Global | No |
| group_concat_max_len | Yes | Yes | Yes | Both | Yes |
| have_archive | Yes | Global | No | ||
| have_bdb | Yes | Global | No | ||
| have_blackhole_engine | Yes | Global | No | ||
| have_compress | Yes | Global | No | ||
| have_crypt | Yes | Global | No | ||
| have_csv | Yes | Global | No | ||
| have_example_engine | Yes | Global | No | ||
| have_geometry | Yes | Global | No | ||
| have_innodb | Yes | Global | No | ||
| have_isam | Yes | Global | No | ||
| have_merge_engine | Yes | Global | No | ||
| have_ndbcluster | Yes | Global | No | ||
| have_openssl | Yes | Global | No | ||
| have_query_cache | Yes | Global | No | ||
| have_raid | Yes | Global | No | ||
| have_rtree_keys | Yes | Global | No | ||
| have_symlink | Yes | Global | No | ||
| identity | Yes | Session | Yes | ||
| init_connect | Yes | Yes | Yes | Global | Yes |
| init-file | Yes | Yes | No | ||
| - Variable: init_file | Yes | Global | No | ||
| init_slave | Yes | Yes | Yes | Global | Yes |
| innodb_additional_mem_pool_size | Yes | Yes | Yes | Global | No |
| innodb_autoextend_increment | Yes | Yes | Yes | Global | Yes |
| innodb_buffer_pool_awe_mem_mb | Yes | Yes | Yes | Global | No |
| innodb_buffer_pool_size | Yes | Yes | Yes | Global | No |
| innodb_data_file_path | Yes | Yes | Yes | Global | No |
| innodb_data_home_dir | Yes | Yes | Yes | Global | No |
| innodb_fast_shutdown | Yes | Yes | Yes | Global | Yes |
| innodb_file_io_threads | Yes | Yes | Yes | Global | No |
| innodb_file_per_table | Yes | Yes | Yes | Global | No |
| innodb_flush_log_at_trx_commit | Yes | Yes | Yes | Global | Yes |
| innodb_flush_method | Yes | Yes | Yes | Global | No |
| innodb_force_recovery | Yes | Yes | Yes | Global | No |
| innodb_lock_wait_timeout | Yes | Yes | Yes | Global | No |
| innodb_locks_unsafe_for_binlog | Yes | Yes | Yes | Global | No |
| innodb_log_arch_dir | Yes | Yes | Yes | Global | No |
| innodb_log_archive | Yes | Yes | Yes | Global | No |
| innodb_log_buffer_size | Yes | Yes | Yes | Global | No |
| innodb_log_file_size | Yes | Yes | Yes | Global | No |
| innodb_log_files_in_group | Yes | Yes | Yes | Global | No |
| innodb_log_group_home_dir | Yes | Yes | Yes | Global | No |
| innodb_max_dirty_pages_pct | Yes | Yes | Yes | Global | Yes |
| innodb_max_purge_lag | Yes | Yes | Yes | Global | Yes |
| innodb_mirrored_log_groups | Yes | Yes | Yes | Global | No |
| innodb_open_files | Yes | Yes | Yes | Global | No |
| innodb_table_locks | Yes | Yes | Yes | Both | Yes |
| innodb_thread_concurrency | Yes | Yes | Yes | Global | Yes |
| insert_id | Yes | Session | Yes | ||
| interactive_timeout | Yes | Yes | Yes | Both | Yes |
| join_buffer_size | Yes | Yes | Yes | Both | Yes |
| key_buffer_size | Yes | Yes | Yes | Global | Yes |
| key_cache_age_threshold | Yes | Yes | Yes | Global | Yes |
| key_cache_block_size | Yes | Yes | Yes | Global | Yes |
| key_cache_division_limit | Yes | Yes | Yes | Global | Yes |
| language | Yes | Yes | Yes | Global | No |
| last_insert_id | Yes | Session | Yes | ||
| lc_time_names | Yes | Both | Yes | ||
| license | Yes | Global | No | ||
| local_infile | Yes | Global | Yes | ||
| locked_in_memory | Yes | Global | No | ||
| log | Yes | Yes | Yes | Global | No |
| log_bin | Yes | Global | No | ||
| log-bin | Yes | Yes | Yes | Global | No |
| log-error | Yes | Yes | No | ||
| - Variable: log_error | Yes | Global | No | ||
| log-slave-updates | Yes | Yes | No | ||
| - Variable: log_slave_updates | Yes | Global | No | ||
| log_slave_updates | Yes | Yes | Yes | Global | No |
| log-slow-queries | Yes | Yes | No | ||
| - Variable: log_slow_queries | Yes | Global | No | ||
| log-warnings | Yes | Yes | Yes | ||
| - Variable: log_warnings | Yes | Both | Yes | ||
| long_query_time | Yes | Yes | Yes | Both | Yes |
| low-priority-updates | Yes | Yes | Yes | ||
| - Variable: low_priority_updates | Yes | Both | Yes | ||
| lower_case_file_system | Yes | Global | No | ||
| lower_case_table_names | Yes | Yes | Yes | Global | No |
| max_allowed_packet | Yes | Yes | Yes | Global | Yes |
| max_binlog_cache_size | Yes | Yes | Yes | Global | Yes |
| max_binlog_size | Yes | Yes | Yes | Global | Yes |
| max_connect_errors | Yes | Yes | Yes | Global | Yes |
| max_connections | Yes | Yes | Yes | Global | Yes |
| max_delayed_threads | Yes | Yes | Yes | Both | Yes |
| max_error_count | Yes | Yes | Yes | Both | Yes |
| max_heap_table_size | Yes | Yes | Yes | Both | Yes |
| max_insert_delayed_threads | Yes | Both | Yes | ||
| max_join_size | Yes | Yes | Yes | Both | Yes |
| max_length_for_sort_data | Yes | Yes | Yes | Both | Yes |
| max_prepared_stmt_count | Yes | Global | Yes | ||
| max_relay_log_size | Yes | Yes | Yes | Global | Yes |
| max_seeks_for_key | Yes | Yes | Yes | Both | Yes |
| max_sort_length | Yes | Yes | Yes | Both | Yes |
| max_user_connections | Yes | Yes | Yes | Global | Yes |
| max_write_lock_count | Yes | Yes | Yes | Global | Yes |
| myisam_data_pointer_size | Yes | Yes | Yes | Global | Yes |
| myisam_max_extra_sort_file_size | Yes | Yes | Yes | Global | No |
| myisam_max_sort_file_size | Yes | Yes | Yes | Global | Yes |
| myisam_recover_options | Yes | Global | No | ||
| myisam_repair_threads | Yes | Yes | Yes | Both | Yes |
| myisam_sort_buffer_size | Yes | Yes | Yes | Both | Yes |
| myisam_stats_method | Yes | Yes | Yes | Both | Yes |
| named_pipe | Yes | Global | No | ||
| ndb_autoincrement_prefetch_sz | Yes | Yes | Yes | Both | Yes |
| ndb_cache_check_time | Yes | Yes | Yes | Global | Yes |
| ndb_force_send | Yes | Yes | Yes | Both | Yes |
| ndb_index_stat_cache_entries | Yes | Yes | Yes | Both | Yes |
| ndb_index_stat_enable | Yes | Yes | Yes | Both | Yes |
| ndb_index_stat_update_freq | Yes | Yes | Yes | Both | Yes |
| ndb_optimized_node_selection | Yes | Yes | Yes | Global | No |
| ndb_use_exact_count | Yes | Both | Yes | ||
| ndb_use_transactions | Yes | Yes | Yes | Both | Yes |
| net_buffer_length | Yes | Yes | Yes | Both | Yes |
| net_read_timeout | Yes | Yes | Yes | Both | Yes |
| net_retry_count | Yes | Yes | Yes | Both | Yes |
| net_write_timeout | Yes | Yes | Yes | Both | Yes |
| new | Yes | Yes | Yes | Both | Yes |
| old_passwords | Yes | Both | Yes | ||
| open-files-limit | Yes | Yes | No | ||
| - Variable: open_files_limit | Yes | Global | No | ||
| pid-file | Yes | Yes | No | ||
| - Variable: pid_file | Yes | Global | No | ||
| plugin_dir | Yes | Yes | Yes | Global | No |
| port | Yes | Yes | Yes | Global | No |
| preload_buffer_size | Yes | Yes | Yes | Both | Yes |
| prepared_stmt_count | Yes | Both | No | ||
| protocol_version | Yes | Global | No | ||
| pseudo_thread_id | Yes | Session | Yes | ||
| query_alloc_block_size | Yes | Yes | Yes | Both | Yes |
| query_cache_limit | Yes | Yes | Yes | Global | Yes |
| query_cache_min_res_unit | Yes | Yes | Yes | Global | Yes |
| query_cache_size | Yes | Yes | Yes | Global | Yes |
| query_cache_type | Yes | Yes | Yes | Both | Yes |
| query_cache_wlock_invalidate | Yes | Yes | Yes | Both | Yes |
| query_prealloc_size | Yes | Yes | Yes | Both | Yes |
| rand_seed1 | Yes | Session | Yes | ||
| rand_seed2 | Yes | Session | Yes | ||
| range_alloc_block_size | Yes | Yes | Yes | Both | Yes |
| read_buffer_size | Yes | Yes | Yes | Both | Yes |
| read_only | Yes | Yes | Yes | Global | Yes |
| read_rnd_buffer_size | Yes | Yes | Yes | Both | Yes |
| relay-log | Yes | Yes | No | ||
| - Variable: relay_log | Yes | Global | No | ||
| relay-log-index | Yes | Yes | No | ||
| - Variable: relay_log_index | Yes | Global | No | ||
| relay_log_index | Yes | Yes | Yes | Global | No |
| relay_log_info_file | Yes | Yes | Yes | Global | No |
| relay_log_purge | Yes | Yes | Yes | Global | Yes |
| relay_log_space_limit | Yes | Yes | Yes | Global | No |
| rpl_recovery_rank | Yes | Global | Yes | ||
| safe-show-database | Yes | Yes | Yes | ||
| - Variable: safe_show_database | Yes | Global | Yes | ||
| secure-auth | Yes | Yes | Yes | ||
| - Variable: secure_auth | Yes | Global | Yes | ||
| server-id | Yes | Yes | Yes | ||
| - Variable: server_id | Yes | Global | Yes | ||
| shared_memory | Yes | Yes | Yes | Global | No |
| shared_memory_base_name | Yes | Global | No | ||
| skip_external_locking | Yes | Yes | Yes | Global | No |
| skip-networking | Yes | Yes | No | ||
| - Variable: skip_networking | Yes | Global | No | ||
| skip-show-database | Yes | Yes | No | ||
| - Variable: skip_show_database | Yes | Global | No | ||
| skip-sync-bdb-logs | Yes | Yes | Yes | Global | No |
| slave_compressed_protocol | Yes | Yes | Yes | Global | Yes |
| slave-load-tmpdir | Yes | Yes | No | ||
| - Variable: slave_load_tmpdir | Yes | Global | No | ||
| slave-net-timeout | Yes | Yes | Yes | ||
| - Variable: slave_net_timeout | Yes | Global | Yes | ||
| slave-skip-errors | Yes | Yes | No | ||
| - Variable: slave_skip_errors | Yes | Global | No | ||
| slave_transaction_retries | Yes | Yes | Yes | Global | Yes |
| slow_launch_time | Yes | Yes | Yes | Global | Yes |
| socket | Yes | Yes | Yes | Global | No |
| sort_buffer_size | Yes | Yes | Yes | Both | Yes |
| sql_auto_is_null | Yes | Session | Yes | ||
| sql_big_selects | Yes | Session | Yes | ||
| sql_big_tables | Yes | Session | Yes | ||
| sql_buffer_result | Yes | Session | Yes | ||
| sql_log_bin | Yes | Session | Yes | ||
| sql_log_off | Yes | Session | Yes | ||
| sql_log_update | Yes | Session | Yes | ||
| sql_low_priority_updates | Yes | Both | Yes | ||
| sql_max_join_size | Yes | Both | Yes | ||
| sql-mode | Yes | Yes | Yes | ||
| - Variable: sql_mode | Yes | Both | Yes | ||
| sql_notes | Yes | Session | Yes | ||
| sql_quote_show_create | Yes | Session | Yes | ||
| sql_safe_updates | Yes | Session | Yes | ||
| sql_select_limit | Yes | Both | Yes | ||
| sql_slave_skip_counter | Yes | Global | Yes | ||
| sql_warnings | Yes | Session | Yes | ||
| ssl-ca | Yes | Yes | No | ||
| - Variable: ssl_ca | Yes | Global | No | ||
| ssl-capath | Yes | Yes | No | ||
| - Variable: ssl_capath | Yes | Global | No | ||
| ssl-cert | Yes | Yes | No | ||
| - Variable: ssl_cert | Yes | Global | No | ||
| ssl-cipher | Yes | Yes | No | ||
| - Variable: ssl_cipher | Yes | Global | No | ||
| ssl-key | Yes | Yes | No | ||
| - Variable: ssl_key | Yes | Global | No | ||
| storage_engine | Yes | Both | Yes | ||
| sync-bdb-logs | Yes | Yes | Yes | Global | No |
| sync_binlog | Yes | Yes | Yes | Global | Yes |
| sync_frm | Yes | Yes | Yes | Global | Yes |
| system_time_zone | Yes | Global | No | ||
| table_cache | Yes | Yes | Yes | Global | Yes |
| table_type | Yes | Both | Yes | ||
| thread_cache_size | Yes | Yes | Yes | Global | Yes |
| thread_concurrency | Yes | Yes | Yes | Global | No |
| thread_stack | Yes | Yes | Yes | Global | No |
| time_format | Yes | Both | No | ||
| time_zone | Yes | Both | Yes | ||
| timestamp | Yes | Session | Yes | ||
| tmp_table_size | Yes | Yes | Yes | Both | Yes |
| tmpdir | Yes | Yes | Yes | Global | No |
| transaction_alloc_block_size | Yes | Yes | Yes | Both | Yes |
| transaction_prealloc_size | Yes | Yes | Yes | Both | Yes |
| tx_isolation | Yes | Both | Yes | ||
| unique_checks | Yes | Session | Yes | ||
| version | Yes | Global | No | ||
| version_comment | Yes | Global | No | ||
| version_compile_machine | Yes | Global | No | ||
| version_compile_os | Yes | Global | No | ||
| wait_timeout | Yes | Yes | Yes | Both | Yes |
| warning_count | Yes | Session | No | ||
|
[a] This option is dynamic, but only the server should set this information. You should not set the value of this variable manually. [b] This option is dynamic, but only the server should set this information. You should not set the value of this variable manually. | |||||
For additional system variable information, see these sections:
Section 5.1.4, “Using System Variables”, discusses the syntax for setting and displaying system variable values.
Section 5.1.4.2, “Dynamic System Variables”, lists the variables that can be set at runtime.
Information on tuning system variables can be found in Section 7.8.2, “Tuning Server Parameters”.
Section 13.2.4, “InnoDB Startup Options and System Variables”, lists
InnoDB system variables.
Section 15.3.4.3, “MySQL Cluster System Variables”, lists system variables which are specific to MySQL Cluster.
For information on server system variables specific to replication, see Section 14.8, “Replication and Binary Logging Options and Variables”.
Some of the following variable descriptions refer to
“enabling” or “disabling” a variable.
These variables can be enabled with the
SET
statement by setting them to ON or
1, or disabled by setting them to
OFF or 0. However, to set
such a variable on the command line or in an option file, you
must set it to 1 or 0;
setting it to ON or OFF
will not work. For example, on the command line,
--delay_key_write=1 works but
--delay_key_write=ON does not.
Some system variables control the size of buffers or caches. For a given buffer, the server might need to allocate internal data structures. These structures typically are allocated from the total memory allocated to the buffer, and the amount of space required might be platform dependent. This means that when you assign a value to a system variable that controls a buffer size, the amount of space actually available might differ from the value assigned. In some cases, the amount might be less than the value assigned. It is also possible that the server will adjust a value upward. For example, if you assign a value of 0 to a variable for which the minimal value is 1024, the server will set the value to 1024.
Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified.
Some system variables take file name values. Unless otherwise
specified, the default file location is the data directory if the
value is a relative path name. To specify the location explicitly,
use an absolute path name. Suppose that the data directory is
/var/mysql/data. If a file-valued variable is
given as a relative path name, it will be located under
/var/mysql/data. If the value is an absolute
path name, its location is as given by the path name.
This is ON if mysqld was
started with --ansi. See
Section 1.9.3, “Running MySQL in ANSI Mode”. This variable was added in MySQL
3.23.6 and removed in 3.23.41. See the description for
sql_mode.
Depending on the network configuration of your system and the
Host values for your accounts, clients may
need to connect using an explicit --host
option, such as --host=localhost or
--host=127.0.0.1.
The autocommit mode. If set to 1, all changes to a table take
effect immediately. If set to 0, you must use
COMMIT to accept a transaction
or ROLLBACK
to cancel it. If autocommit
is 0 and you change it to 1, MySQL performs an automatic
COMMIT of any open transaction.
Another way to begin a transaction is to use a
START
TRANSACTION or
BEGIN
statement. See Section 12.3.1, “START TRANSACTION, COMMIT, and ROLLBACK Syntax”.
By default, client connections begin with
autocommit set to 1. To cause
clients to begin with a default of 0, set the server's
init_connect system variable.
See the description of that variable for instructions that
show how to do this.
The number of outstanding connection requests MySQL can have.
This comes into play when the main MySQL thread gets very many
connection requests in a very short time. It then takes some
time (although very little) for the main thread to check the
connection and start a new thread. The
back_log value indicates how
many requests can be stacked during this short time before
MySQL momentarily stops answering new requests. You need to
increase this only if you expect a large number of connections
in a short period of time.
In other words, this value is the size of the listen queue for
incoming TCP/IP connections. Your operating system has its own
limit on the size of this queue. The manual page for the Unix
listen() system call should have more
details. Check your OS documentation for the maximum value for
this variable. back_log
cannot be set higher than your operating system limit.
The MySQL installation base directory. This variable can be
set with the --basedir option.
Relative path names for other variables usually are resolved
relative to the base directory.
The size of the buffer that is allocated for caching indexes
and rows for BDB tables. If you do not use
BDB tables, you should start
mysqld with
--skip-bdb to not allocate
memory for this cache. This variable was added in MySQL
3.23.14.
The base directory for BDB tables. This
should be assigned the same value as the
datadir variable. This
variable was added in MySQL 3.23.14.
The size of the buffer that is allocated for caching indexes
and rows for BDB tables. If you do not use
BDB tables, you should set this to 0 or
start mysqld with
--skip-bdb in order not to
allocate memory for this cache. This variable was added in
MySQL 3.23.31.
The directory where the BDB storage engine
writes its log files. This variable can be set with the
--bdb-logdir option. This
variable was added in MySQL 3.23.14.
The maximum number of locks that can be active for a
BDB table (10,000 by default). You should
increase this value if errors such as the following occur when
you perform long transactions or when
mysqld has to examine many rows to
calculate a query:
bdb: Lock table is out of available locks Got error 12 from ...
This variable was added in MySQL 3.23.29.
This is ON if you are using
--bdb-shared-data to start
Berkeley DB in multi-process mode. (Do not use
DB_PRIVATE when initializing Berkeley DB.)
This variable was added in MySQL 3.23.29.
The BDB temporary file directory. This
variable was added in MySQL 3.23.14.
See the description for
version_bdb.
If set to 1, all temporary tables are stored on disk rather
than in memory. This is a little slower, but the error
The table does not occur for
tbl_name is
fullSELECT operations that require
a large temporary table. The default value for a new
connection is 0 (use in-memory temporary tables). As of MySQL
4.0, you should normally never need to set this variable,
because MySQL automatically converts in-memory tables to
disk-based tables as necessary.
This variable was formerly named
sql_big_tables.
The size of the cache to hold the SQL statements for the
binary log during a transaction. A binary log cache is
allocated for each client if the server supports any
transactional storage engines and, starting from MySQL 4.1.2,
if the server has the binary log enabled
(--log-bin option). If you
often use large, multiple-statement transactions, you can
increase this cache size to get more performance. The
Binlog_cache_use and
Binlog_cache_disk_use status
variables can be useful for tuning the size of this variable.
This variable was added in MySQL 3.23.29. See
Section 5.3.4, “The Binary Log”.
MyISAM uses a special tree-like cache to
make bulk inserts faster for
INSERT ...
SELECT, INSERT ... VALUES (...), (...),
..., and
LOAD DATA
INFILE when adding data to nonempty tables. This
variable limits the size of the cache tree in bytes per
thread. Setting it to 0 disables this optimization. The
default value is 8MB. Before MySQL 4.0.3. this variable was
named myisam_bulk_insert_tree_size.
The default character set. This variable was added in MySQL
3.23.3, then removed in MySQL 4.1.1 and replaced by the
various
character_set_
variables.
xxx
The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.
The session value of this variable is set using the character
set requested by the client when the client connects to the
server. (Many clients support a
--default-character-set option to enable this
character set to be specified explicitly. See also
Section 9.1.4, “Connection Character Sets and Collations”.) The global value of the
variable is used to set the session value in cases when the
client-requested value is unknown or not available, or the
server is configured to ignore client requests:
The client is from a version of MySQL older than MySQL 4.1, and thus does not request a character set.
The client requests a character set not known to the
server. For example, a Japanese-enabled client requests
sjis when connecting to a server not
configured with sjis support.
mysqld was started with the
--skip-character-set-client-handshake
option, which causes it to ignore client character set
configuration. This reproduces MySQL 4.0 behavior and is
useful should you wish to upgrade the server without
upgrading all the clients.
ucs2 cannot be used as a client character
set, which means that it also does not work for SET
NAMES or SET CHARACTER SET.
The character set used for literals that do not have a character set introducer and for number-to-string conversion. This variable was added in MySQL 4.1.1.
The character set used by the default database. The server
sets this variable whenever the default database changes. If
there is no default database, the variable has the same value
as character_set_server. This
variable was added in MySQL 4.1.1.
The character set used for returning query results such as result sets or error messages to the client. This variable was added in MySQL 4.1.1.
The server default character set. This variable was added in MySQL 4.1.1.
The character set used by the server for storing identifiers.
The value is always utf8. This variable was
added in MySQL 4.1.1.
The supported character sets. This variable was added in MySQL
3.23.15 and removed in MySQL 4.1.1. (Use
SHOW CHARACTER SET for a list
of character sets.)
The directory where character sets are installed. This variable was added in MySQL 4.1.2.
The collation of the connection character set. This variable was added in MySQL 4.1.1.
The collation used by the default database. The server sets
this variable whenever the default database changes. If there
is no default database, the variable has the same value as
collation_server. This
variable was added in MySQL 4.1.1.
The server default collation. This variable was added in MySQL 4.1.1.
If ON (the default), MySQL permits
INSERT and
SELECT statements to run
concurrently for MyISAM tables that have no
free blocks in the middle of the data file. If
OFF, concurrent inserts are disabled. If
you start mysqld with
--skip-new,
this variable is set to OFF. This variable
was added in MySQL 3.23.7.
See also Section 7.6.3, “Concurrent Inserts”.
The number of seconds that the mysqld
server waits for a connect packet before responding with
Bad handshake. The default value is 5
seconds.
Increasing the
connect_timeout value might
help if clients frequently encounter errors of the form
Lost connection to MySQL server at
'.
XXX', system error:
errno
The current character set mapping that was set by SET
CHARACTER SET. This variable was removed in MySQL
4.1.
The MySQL data directory. This variable can be set with the
--datadir option.
This variable is unused.
This variable is unused.
The default mode value to use for the
WEEK() function. See
Section 11.7, “Date and Time Functions”. This variable is
available as of MySQL 4.0.14.
This option applies only to MyISAM tables.
It can have one of the following values to affect handling of
the DELAY_KEY_WRITE table option that can
be used in CREATE TABLE
statements.
| Option | Description |
|---|---|
OFF | DELAY_KEY_WRITE is ignored. |
ON | MySQL honors any DELAY_KEY_WRITE option specified in
CREATE TABLE
statements. This is the default value. |
ALL | All new opened tables are treated as if they were created with the
DELAY_KEY_WRITE option enabled. |
If DELAY_KEY_WRITE is enabled for a table,
the key buffer is not flushed for the table on every index
update, but only when the table is closed. This speeds up
writes on keys a lot, but if you use this feature, you should
add automatic checking of all MyISAM tables
by starting the server with the
--myisam-recover option (for
example,
--myisam-recover=BACKUP,FORCE).
See Section 5.1.2, “Server Command Options”, and
Section 13.1.1, “MyISAM Startup Options”.
If you enable external locking with
--external-locking, there is
no protection against index corruption for tables that use
delayed key writes.
This variable was added in MySQL 3.23.8.
After inserting
delayed_insert_limit delayed
rows, the INSERT DELAYED
handler thread checks whether there are any
SELECT statements pending. If
so, it permits them to execute before continuing to insert
delayed rows.
How many seconds an INSERT
DELAYED handler thread should wait for
INSERT statements before
terminating.
This is a per-table limit on the number of rows to queue when
handling INSERT DELAYED
statements. If the queue becomes full, any client that issues
an INSERT DELAYED statement
waits until there is room in the queue again.
The number of errors that resulted from the last statement that generated messages. This variable is read only. See Section 12.4.5.11, “SHOW ERRORS Syntax”.
This variable was added in MySQL 4.1.0.
The number of days for automatic binary log file removal. The default is 0, which means “no automatic removal.” Possible removals happen at startup and when the binary log is flushed. Log flushing occurs as indicated in Section 5.3, “MySQL Server Logs”. This variable was added in MySQL 4.1.0.
To remove binary log files manually, use the
PURGE BINARY LOGS statement.
See Section 12.5.1.1, “PURGE BINARY LOGS Syntax”.
If ON, the server flushes (synchronizes)
all changes to disk after each SQL statement. Normally, MySQL
does a write of all changes to disk only after each SQL
statement and lets the operating system handle the
synchronizing to disk. See Section B.5.4.2, “What to Do If MySQL Keeps Crashing”. This
variable is set to ON if you start
mysqld with the
--flush option. This variable
was added in MySQL 3.22.9.
If this is set to a nonzero value, all tables are closed every
flush_time seconds to free up
resources and synchronize unflushed data to disk. This option
is best used only on Windows 9x or Me, or on systems with
minimal resources. This variable was added in MySQL 3.22.18.
If set to 1 (the default), foreign key constraints for
InnoDB tables are checked. If set to 0,
they are ignored. Disabling foreign key checking can be useful
for reloading InnoDB tables in an order
different from that required by their parent/child
relationships. This variable was added in MySQL 3.23.52. See
Section 13.2.5.4, “FOREIGN KEY Constraints”.
Setting foreign_key_checks to
0 also affects data definition statements:
DROP DATABASE drops a database
even if it contains tables that have foreign keys that are
referred to by tables outside the database, and
DROP TABLE drops tables that
have foreign keys that are referred to by other tables.
Setting foreign_key_checks
to 1 does not trigger a scan of the existing table data.
Therefore, rows added to the table while
foreign_key_checks = 0 will
not be verified for consistency.
The list of operators supported by boolean full-text searches
performed using IN BOOLEAN MODE. See
Section 11.9.2, “Boolean Full-Text Searches”. This variable was added as
a read-only variable in MySQL 4.0.1. It can be modified as of
MySQL 4.1.2.
The default variable value is
'+ -><()~*:""&|'. The rules
for changing the value are as follows:
Operator function is determined by position within the string.
The replacement value must be 14 characters.
Each character must be an ASCII nonalphanumeric character.
Either the first or second character must be a space.
No duplicates are permitted except the phrase quoting operators in positions 11 and 12. These two characters are not required to be the same, but they are the only two that may be.
Positions 10, 13, and 14 (which by default are set to
“:”,
“&”, and
“|”) are reserved for
future extensions.
The maximum length of the word to be included in a
FULLTEXT index. This variable was added in
MySQL 4.0.0.
FULLTEXT indexes must be rebuilt after
changing this variable. Use REPAIR TABLE
.
tbl_name QUICK
The minimum length of the word to be included in a
FULLTEXT index. This variable was added in
MySQL 4.0.0.
FULLTEXT indexes must be rebuilt after
changing this variable. Use REPAIR TABLE
.
tbl_name QUICK
The number of top matches to use for full-text searches
performed using WITH QUERY EXPANSION. This
variable was added in MySQL 4.1.1.
The file from which to read the list of stopwords for
full-text searches. The server looks for the file in the data
directory unless an absolute path name is given to specify a
different directory. All the words from the file are used;
comments are not honored. By default, a
built-in list of stopwords is used (as defined in the
myisam/ft_static.c file). Setting this
variable to the empty string ('') disables
stopword filtering. See also
Section 11.9.4, “Full-Text Stopwords”. This variable was added
in MySQL 4.0.10.
FULLTEXT indexes must be rebuilt after
changing this variable or the contents of the stopword file.
Use REPAIR TABLE
.
tbl_name QUICK
The maximum permitted result length in bytes for the
GROUP_CONCAT() function. The
default is 1024. This variable was added in MySQL 4.1.0.
YES if mysqld supports
ARCHIVE tables, NO if
not. This variable was added in MySQL 4.1.3.
YES if mysqld supports
BDB tables. DISABLED if
--skip-bdb is used. This
variable was added in MySQL 3.23.30.
YES if mysqld supports
BLACKHOLE tables, NO if
not. This variable was added in MySQL 4.1.11.
YES if the zlib
compression library is available to the server,
NO if not. If not, the
COMPRESS() and
UNCOMPRESS() functions cannot
be used. This variable was added in MySQL 4.1.1.
YES if the crypt()
system call is available to the server, NO
if not. If not, the ENCRYPT()
function cannot be used. This variable was added in MySQL
4.0.10.
YES if mysqld supports
ARCHIVE tables, NO if
not. This variable was added in MySQL 4.1.4.
YES if mysqld supports
EXAMPLE tables, NO if
not. This variable was added in MySQL 4.1.4.
YES if the server supports spatial data
types, NO if not. This variable was added
in MySQL 4.1.3.
YES if mysqld supports
InnoDB tables. DISABLED
if
--skip-innodb
is used. This variable was added in MySQL 3.23.37.
YES if mysqld supports
ISAM tables. DISABLED if
--skip-isam is used. This
variable was added in MySQL 3.23.30.
YES if mysqld supports
MERGE tables. DISABLED
if --skip-merge is used. This
variable was added in MySQL 4.1.21.
YES if mysqld supports
SSL (encryption) connections, NO if not.
This variable was added in MySQL 3.23.43.
YES if mysqld supports
the query cache, NO if not. This variable
was added in MySQL 4.0.2.
YES if mysqld supports
the RAID option, NO if
not. This variable was added in MySQL 3.23.30.
YES if RTREE indexes are
available, NO if not. (These are used for
spatial indexes in MyISAM tables.) This
variable was added in MySQL 4.1.3.
YES if symbolic link support is enabled,
NO if not. This is required on Unix for
support of the DATA DIRECTORY and
INDEX DIRECTORY table options, and on
Windows for support of data directory symlinks.
This variable was added in MySQL 4.0.0.
This variable is a synonym for the
last_insert_id variable. It
exists for compatibility with other database systems. As of
MySQL 3.23.25, you can read its value with SELECT
@@identity. As of MySQL 4.0.3, you can also set its
value with SET identity.
A string to be executed by the server for each client that
connects. The string consists of one or more SQL statements.
To specify multiple statements, separate them by semicolon
characters. For example, each client begins by default with
autocommit mode enabled. There is no global system variable to
specify that autocommit should be disabled by default, but
init_connect can be used to
achieve the same effect:
SET GLOBAL init_connect='SET autocommit=0';
This variable can also be set on the command line or in an option file. To set the variable as just shown using an option file, include these lines:
[mysqld] init_connect='SET autocommit=0'
The content of init_connect
is not executed for users that have the
SUPER privilege. This is done
so that an erroneous value for
init_connect does not prevent
all clients from connecting. For example, the value might
contain a statement that has a syntax error, thus causing
client connections to fail. Not executing
init_connect for users that
have the SUPER privilege
enables them to open a connection and fix the
init_connect value.
This variable was added in MySQL 4.1.2.
The name of the file specified with the
--init-file option when you
start the server. This should be a file containing SQL
statements that you want the server to execute when it starts.
Each statement must be on a single line and should not include
comments. No statement terminator such as
;, \g, or
\G should be given at the end of each
statement. This variable was added in MySQL 3.23.2.
innodb_
xxx
InnoDB system variables are listed in
Section 13.2.4, “InnoDB Startup Options and System Variables”.
The value to be used by the following
INSERT or
ALTER TABLE statement when
inserting an AUTO_INCREMENT value. This is
mainly used with the binary log.
The number of seconds the server waits for activity on an
interactive connection before closing it. An interactive
client is defined as a client that uses the
CLIENT_INTERACTIVE option to
mysql_real_connect(). See also
wait_timeout.
The minimum size of the buffer that is used for plain index
scans, range index scans, and joins that do not use indexes
and thus perform full table scans. Normally, the best way to
get fast joins is to add indexes. Increase the value of
join_buffer_size to get a
faster full join when adding indexes is not possible. One join
buffer is allocated for each full join between two tables. For
a complex join between several tables for which indexes are
not used, multiple join buffers might be necessary. There is
no gain from setting the buffer larger than required to hold
each matching row, and all joins allocate at least the minimum
size, so use caution in setting this variable to a large value
globally. It is better to keep the global setting small and
change to a larger setting only in sessions that are doing
large joins. Memory allocation time can cause substantial
performance drops if the global size is larger than needed by
most queries that use it.
For additional information about join buffering, see Section 7.3.1.6, “Nested-Loop Join Algorithms”.
Index blocks for MyISAM and
ISAM tables are buffered and are shared by
all threads. key_buffer_size
is the size of the buffer used for index blocks. The key
buffer is also known as the key cache.
The maximum permissible setting for
key_buffer_size is 4GB. The
effective maximum size might be less, depending on your
available physical RAM and per-process RAM limits imposed by
your operating system or hardware platform. The value of this
variable indicates the amount of memory requested. Internally,
the server allocates as much memory as possible up to this
amount, but the actual allocation might be less.
You can increase the value to get better index handling for
all reads and multiple writes; on a system whose primary
function is to run MySQL using the
MyISAM storage engine, 25% of the
machine's total memory is an acceptable value for this
variable. However, you should be aware that, if you make the
value too large (for example, more than 50% of the
machine's total memory), your system might start to page
and become extremely slow. This is because MySQL relies on the
operating system to perform file system caching for data
reads, so you must leave some room for the file system cache.
You should also consider the memory requirements of any other
storage engines that you may be using in addition to
MyISAM.
For even more speed when writing many rows at the same time,
use LOCK TABLES. See
Section 7.3.2.1, “Speed of INSERT Statements”.
You can check the performance of the key buffer by issuing a
SHOW STATUS statement and
examining the
Key_read_requests,
Key_reads,
Key_write_requests, and
Key_writes status variables.
(See Section 12.4.5, “SHOW Syntax”.) The
Key_reads/Key_read_requests ratio should
normally be less than 0.01. The
Key_writes/Key_write_requests ratio is
usually near 1 if you are using mostly updates and deletes,
but might be much smaller if you tend to do updates that
affect many rows at the same time or if you are using the
DELAY_KEY_WRITE table option.
The fraction of the key buffer in use can be determined using
key_buffer_size in
conjunction with the
Key_blocks_unused status
variable and the buffer block size. From MySQL 4.1.1 on, the
buffer block size is available from the
key_cache_block_size server
variable. The fraction of the buffer in use is:
1 - ((Key_blocks_unused * key_cache_block_size) / key_buffer_size)
This value is an approximation because some space in the key buffer is allocated internally for administrative structures. Factors that influence the amount of overhead for these structures include block size and pointer size. As block size increases, the percentage of the key buffer lost to overhead tends to decrease. Larger blocks results in a smaller number of read operations (because more keys are obtained per read), but conversely an increase in reads of keys that are not examined (if not all keys in a block are relevant to a query).
Before MySQL 4.1.1, key cache blocks are 1024 bytes, and
before MySQL 4.1.2,
Key_blocks_unused is
unavailable. The
Key_blocks_used variable can
be used as follows to determine the fraction of the key buffer
in use:
(Key_blocks_used × 1024) / key_buffer_size
However, Key_blocks_used
indicates the maximum number of blocks that have ever been in
use at once, so this formula does not necessarily represent
the current fraction of the buffer that is in use.
As of MySQL 4.1, it is possible to create multiple
MyISAM key caches. The size limit of 4GB
applies to each cache individually, not as a group. See
Section 7.5.1, “The MyISAM Key Cache”.
This value controls the demotion of buffers from the hot sublist of a key cache to the warm sublist. Lower values cause demotion to happen more quickly. The minimum value is 100. The default value is 300. This variable was added in MySQL 4.1.1. See Section 7.5.1, “The MyISAM Key Cache”.
The size in bytes of blocks in the key cache. The default value is 1024. This variable was added in MySQL 4.1.1. See Section 7.5.1, “The MyISAM Key Cache”.
The division point between the hot and warm sublists of the key cache buffer list. The value is the percentage of the buffer list to use for the warm sublist. Permissible values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See Section 7.5.1, “The MyISAM Key Cache”.
The directory where error messages are located. See Section 9.3, “Setting the Error Message Language”.
Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.
The value to be returned from
LAST_INSERT_ID(). This is
stored in the binary log when you use
LAST_INSERT_ID() in a statement
that updates a table. Setting this variable does not update
the value returned by the
mysql_insert_id() C API
function.
This variable specifies the locale that controls the language
used to display day and month names and abbreviations. This
variable affects the output from the
DATE_FORMAT(),
DAYNAME() and
MONTHNAME() functions. Locale
names are POSIX-style values such as
'ja_JP' or 'pt_BR'. The
default value is 'en_US' regardless of your
system's locale setting. For further information, see
Section 9.8, “MySQL Server Locale Support”. This variable was added in
MySQL 4.1.21.
The type of license the server has. This variable was added in MySQL 4.0.19.
Whether LOCAL is supported for
LOAD DATA
INFILE statements. See
Section 5.4.5, “Security Issues with LOAD DATA LOCAL”. This variable was added in
MySQL 4.0.3.
Whether mysqld was locked in memory with
--memlock. This variable was
added in MySQL 3.23.25.
Whether logging of all statements to the general query log is enabled. See Section 5.3.2, “The General Query Log”.
The location of the error log. This variable was added in MySQL 4.0.10.
Whether slow queries should be logged. “Slow” is
determined by the value of the
long_query_time variable.
This variable was added in MySQL 4.0.2. See
Section 5.3.5, “The Slow Query Log”.
Whether the update log is enabled. This variable was added in MySQL 3.22.18. Note that the binary log is preferable to the update log, which is unavailable as of MySQL 5.0. See Section 5.3.3, “The Update Log”.
Whether to produce additional warning messages. This variable was added in MySQL 4.0.3. It is enabled by default as of MySQL 4.0.19 and 4.1.2. As of MySQL 4.0.21 and 4.1.3, the variable can take values greater than 1 and aborted connections are not logged to the error log unless the value is greater than 1.
If a query takes longer than this many seconds, the server
increments the Slow_queries
status variable. If you are using the
--log-slow-queries option, the
query is logged to the slow query log file. This value is
measured in real time, not CPU time, so a query that is under
the threshold on a lightly loaded system might be above the
threshold on a heavily loaded one. The minimum value is 1. The
default is 10. See Section 5.3.5, “The Slow Query Log”.
If set to 1, all
INSERT,
UPDATE,
DELETE, and LOCK TABLE
WRITE statements wait until there is no pending
SELECT or LOCK TABLE
READ on the affected table. This affects only
storage engines that use only table-level locking (such as
MyISAM, MEMORY, and
MERGE). Before MySQL 3.22.5, this variable
was named sql_low_priority_updates.
This variable describes the case sensitivity of file names on
the file system where the data directory is located.
OFF means file names are case sensitive,
ON means they are not case sensitive. This
variable is read only because it reflects a file system
attribute and setting it would have no effect on the file
system. This variable was added in MySQL 4.0.19.
If set to 0, table names are stored as specified and comparisons are case sensitive. If set to 1, table names are stored in lowercase on disk and comparisons are not case sensitive. This variable was added in MySQL 3.23.6. If set to 2 (new in 4.0.18), table names are stored as given but compared in lowercase. From MySQL 4.0.2, this option also applies to database names. From 4.1.1, it also applies to table aliases. For additional information, see Section 8.2.2, “Identifier Case Sensitivity”.
You should not set this variable to 0 if
you are running MySQL on a system that does not have
case-sensitive file names (such as Windows or Mac OS X). If
you set this variable to 0 on such a system and access
MyISAM tablenames using different
lettercases, index corruption may result. New in
4.0.18: If this variable is not set at startup and
the file system on which the data directory is located does
not have case-sensitive file names, MySQL automatically sets
lower_case_table_names to 2.
If you are using InnoDB tables, you should
set this variable to 1 on all platforms to force names to be
converted to lowercase.
The setting of this variable has no effect on replication filtering options. See Section 14.9, “How Servers Evaluate Replication Filtering Rules”, for more information.
You should not use different settings for
lower_case_table_names on replication
masters and slaves. In particular, you should not do this when
the slave uses a case-sensitive file system, as this can cause
replication to fail. For more information, see
Section 14.7.20, “Replication and Variables”.
The maximum size of one packet or any generated/intermediate string.
The packet message buffer is initialized to
net_buffer_length bytes, but
can grow up to
max_allowed_packet bytes when
needed. This value by default is small, to catch large
(possibly incorrect) packets.
You must increase this value if you are using large
BLOB columns or long strings.
It should be as big as the largest
BLOB you want to use. The
protocol limit for
max_allowed_packet is 16MB
before MySQL 4.0 and 1GB thereafter. The value should be a
multiple of 1024; nonmultiples are rounded down to the nearest
multiple.
When you change the message buffer size by changing the value
of the max_allowed_packet
variable, you should also change the buffer size on the client
side if your client program permits it. The default
max_allowed_packet value
built in to the client library is 1GB, but individual client
programs might override this. For example,
mysql and mysqldump have
defaults of 16MB and 24MB, respectively. They also enable you
to change the client-side value by setting
max_allowed_packet on the
command line or in an option file.
If there are more than this number of interrupted connections
from a host, that host is blocked from further connections.
You can unblock blocked hosts with the
FLUSH HOSTS
statement. If a connection is established successfully within
fewer than max_connect_errors
attempts after a previous connection was interrupted, the
error count for the host is cleared to zero. However, once a
host is blocked, the
FLUSH HOSTS
statement is the only way to unblock it.
The maximum permitted number of simultaneous client connections. By default, this is 100. See Section B.5.2.7, “Too many connections”, for more information.
Increasing this value increases the number of file descriptors that mysqld requires. See Section 7.7.2, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.
Do not start more than this number of threads to handle
INSERT DELAYED statements. If
you try to insert data into a new table after all
INSERT DELAYED threads are in
use, the row is inserted as if the DELAYED
attribute was not specified. If you set this to 0, MySQL never
creates a thread to handle DELAYED rows; in
effect, doing so disables DELAYED entirely.
This variable was added in MySQL 3.23.0.
For the SESSION value of this variable, the
only valid values are 0 or the GLOBAL
value.
The maximum number of error, warning, and note messages to be
stored for display by the SHOW
ERRORS or SHOW
WARNINGS statements. This variable was added in
MySQL 4.1.0.
This variable sets the maximum size to which user-created
MEMORY (HEAP) tables are
permitted to grow. The value of the variable is used to
calculate MEMORY table
MAX_ROWS values. Setting this variable has
no effect on any existing MEMORY table,
unless the table is re-created with a statement such as
CREATE TABLE, or altered with
ALTER TABLE or
TRUNCATE TABLE. A server
restart also sets the maximum size of existing
MEMORY tables to the global
max_heap_table_size value.
This variable was added in MySQL 3.23.0.
This variable is also used in conjunction with
tmp_table_size to limit the
size of internal in-memory tables. See
Section 7.7.4, “How MySQL Uses Internal Temporary Tables”.
This variable is a synonym for
max_delayed_threads. It was
added in MySQL 4.0.19.
Do not permit statements that probably need to examine more
than max_join_size rows (for
single-table statements) or row combinations (for
multiple-table statements) or that are likely to do more than
max_join_size disk seeks. By
setting this value, you can catch statements where keys are
not used properly and that would probably take a long time.
Set it if your users tend to perform joins that lack a
WHERE clause, that take a long time, or
that return millions of rows.
Setting this variable to a value other than
DEFAULT resets the value of
sql_big_selects to
0. If you set the
sql_big_selects value again,
the max_join_size variable is
ignored.
If a query result is in the query cache, no result size check is performed, because the result has previously been computed and it does not burden the server to send it to the client.
This variable previously was named
sql_max_join_size.
The cutoff on the size of index values that determines which
filesort algorithm to use. See
Section 7.3.1.7, “ORDER BY Optimization”. This variable was
added in MySQL 4.1.1
This variable limits the total number of prepared statements in the server. It can be used in environments where there is the potential for denial-of-service attacks based on running the server out of memory by preparing huge numbers of statements. If the value is set lower than the current number of prepared statements, existing statements are not affected and can be used, but no new statements can be prepared until the current number drops below the limit. The default value is 16,382. The permissible range of values is from 0 to 1 million. Setting the value to 0 disables prepared statements. This variable was added in MySQL 4.1.19.
If a write by a replication slave to its relay log causes the
current log file size to exceed the value of this variable,
the slave rotates the relay logs (closes the current file and
opens the next one). If
max_relay_log_size is 0, the
server uses max_binlog_size
for both the binary log and the relay log. If
max_relay_log_size is greater
than 0, it constrains the size of the relay log, which enables
you to have different sizes for the two logs. You must set
max_relay_log_size to between
4096 bytes and 1GB (inclusive), or to 0.
The default value is 0. This variable was
added in MySQL 4.0.14. See
Section 14.3, “Replication Implementation Details”.
Limit the assumed maximum number of seeks when looking up rows based on a key. The MySQL optimizer assumes that no more than this number of key seeks are required when searching for matching rows in a table by scanning an index, regardless of the actual cardinality of the index (see Section 12.4.5.13, “SHOW INDEX Syntax”). By setting this to a low value (say, 100), you can force MySQL to prefer indexes instead of table scans.
This variable was added in MySQL 4.0.14.
The number of bytes to use when sorting
BLOB or
TEXT values. Only the first
max_sort_length bytes of each
value are used; the rest are ignored.
The maximum number of temporary tables a client can keep open at the same time. (This variable does not yet do anything.)
The maximum number of simultaneous connections permitted to
any given MySQL user account. A value of 0
means “no limit.” This variable was added in
MySQL 3.23.34.
This variable has only a global form.
After this many write locks, permit some pending read lock requests to be processed in between. This variable was added in MySQL 3.23.7.
The default pointer size in bytes, to be used by
CREATE TABLE for
MyISAM tables when no
MAX_ROWS option is specified. This variable
cannot be less than 2 or larger than 7. The default value is
4. This variable was added in MySQL 4.1.2.
See Section B.5.2.12, “The table is full”.
myisam_max_extra_sort_file_size
If the temporary file used for fast MyISAM
index creation would be larger than using the key cache by the
amount specified here, prefer the key cache method. This is
mainly used to force long character keys in large tables to
use the slower key cache method to create the index. This
variable was added in MySQL 3.23.37.
The value is given in megabytes before 4.0.3 and in bytes thereafter.
The maximum size of the temporary file that MySQL is permitted
to use while re-creating a MyISAM index
(during REPAIR TABLE,
ALTER TABLE, or
LOAD DATA
INFILE). If the file size would be larger than this
value, the index is created using the key cache instead, which
is slower. This variable was added in MySQL 3.23.37.
The value is given in megabytes before 4.0.3 and in bytes thereafter.
The default value is 2GB. If MyISAM index
files exceed this size and disk space is available, increasing
the value may help performance. The space must be available in
the file system containing the directory where the original
index file is located.
The value of the
--myisam-recover option. See
Section 5.1.2, “Server Command Options”. This variable was added in
MySQL 3.23.36.
If this value is greater than 1, MyISAM
table indexes are created in parallel (each index in its own
thread) during the Repair by sorting
process. The default value is 1.
Multi-threaded repair is still beta-quality code. This variable was added in MySQL 4.0.13.
The size of the buffer that is allocated when sorting
MyISAM indexes during a
REPAIR TABLE or when creating
indexes with CREATE INDEX or
ALTER TABLE. This variable was
added in MySQL 3.23.16.
How the server treats NULL values when
collecting statistics about the distribution of index values
for MyISAM tables. This variable has three
possible values, nulls_equal,
nulls_unequal, and
nulls_ignored. For
nulls_equal, all NULL
index values are considered equal and form a single value
group that has a size equal to the number of
NULL values. For
nulls_unequal, NULL
values are considered unequal, and each
NULL forms a distinct value group of size
1. For nulls_ignored,
NULL values are ignored.
The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 7.4.4, “MyISAM Index Statistics Collection”.
Any unique prefix of a valid value may be used to set the value of this variable.
This variable was added in MySQL 4.1.15. For older versions,
the statistics collection method is equivalent to
nulls_equal.
On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.
Each client thread is associated with a connection buffer and
result buffer. Both begin with a size given by
net_buffer_length but are
dynamically enlarged up to
max_allowed_packet bytes as
needed. The result buffer shrinks to
net_buffer_length after each
SQL statement.
This variable should not normally be changed, but if you have
very little memory, you can set it to the expected length of
statements sent by clients. If statements exceed this length,
the connection buffer is automatically enlarged. The maximum
value to which
net_buffer_length can be set
is 1MB.
The number of seconds to wait for more data from a connection
before aborting the read. This timeout applies only to TCP/IP
connections, not to connections made through Unix socket
files, named pipes, or shared memory. When the server is
reading from the client,
net_read_timeout is the
timeout value controlling when to abort. When the server is
writing to the client,
net_write_timeout is the
timeout value controlling when to abort. See also
slave_net_timeout. This
variable was added in MySQL 3.23.20.
If a read on a communication port is interrupted, retry this many times before giving up. This value should be set quite high on FreeBSD because internal interrupts are sent to all threads. This variable was added in MySQL 3.23.7.
The number of seconds to wait for a block to be written to a
connection before aborting the write. This timeout applies
only to TCP/IP connections, not to connections made using Unix
socket files, named pipes, or shared memory. See also
net_read_timeout. This
variable was added in MySQL 3.23.20.
This variable is used in MySQL 4.0 to turn on some 4.1 behaviors. This variable was added in MySQL 4.0.12.
Whether the server should use pre-4.1-style passwords for MySQL user accounts. This variable was added in MySQL 4.1.1.
This is not a variable, but it can be used when setting some variables. It is described in Section 12.4.4, “SET Syntax”.
The number of files that the operating system permits
mysqld to open. This is the real value
permitted by the system and might be different from the value
you gave using the
--open-files-limit option to
mysqld or mysqld_safe.
The value is 0 on systems where MySQL cannot change the number
of open files. This variable was added in MySQL 3.23.20.
The path name of the process ID (PID) file. This variable can
be set with the --pid-file
option. This variable was added in MySQL 3.23.23.
The path name of the plugin directory. This variable was added in MySQL 4.1.25. If the value is nonempty, user-defined function object files must be located in this directory. If the value is empty, the behavior that is used before 4.1.25 applies: The UDF object files must be located in a directory that is searched by your system's dynamic linker.
If the plugin directory is writable by the server, it may be
possible for a user to write executable code to a file in the
directory using SELECT
... INTO DUMPFILE. This can be prevented by making
plugin_dir read only to the
server.
The number of the port on which the server listens for TCP/IP
connections. This variable can be set with the
--port option.
The size of the buffer that is allocated when preloading indexes. This variable was added in MySQL 4.1.1.
The current number of prepared statements. (The maximum number
of statements is given by the
max_prepared_stmt_count
system variable.) This variable was added in MySQL 4.1.19. In
MySQL 4.1.23, it was converted to the global
Prepared_stmt_count status
variable.
The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.
| System Variable | Name | pseudo_thread_id | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | numeric | |
This variable is for internal server use.
The allocation size of memory blocks that are allocated for objects created during statement parsing and execution. If you have problems with memory fragmentation, it might help to increase this parameter. This variable was added in MySQL 4.0.16.
Do not cache results that are larger than this number of bytes. The default value is 1MB. This variable was added in MySQL 4.0.1.
The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in Section 7.5.3.3, “Query Cache Configuration”. This variable is present from MySQL 4.1.
The amount of memory allocated for caching query results. The
default value is 0, which disables the
query cache. The permissible values are multiples of 1024;
other values are rounded down to the nearest multiple. Note
that query_cache_size bytes
of memory are allocated even if
query_cache_type is set to
0. This variable was added in MySQL 4.0.1.
The query cache needs a minimum size of about 40KB to allocate
its structures. (The exact size depends on system
architecture.) If you set the value of
query_cache_size too small, a
warning will occur, as described in
Section 7.5.3.3, “Query Cache Configuration”.
Set the query cache type. Setting the
GLOBAL value sets the type for all clients
that connect thereafter. Individual clients can set the
SESSION value to affect their own use of
the query cache.
| Option | Description |
|---|---|
0 or OFF | Do not cache results in or retrieve results from the query cache. Note
that this does not deallocate the query cache buffer.
To do that, you should set
query_cache_size to
0. |
1 or ON | Cache all cacheable query results except for those that begin with
SELECT SQL_NO_CACHE. |
2 or DEMAND | Cache results only for cacheable queries that begin with SELECT
SQL_CACHE. |
This variable defaults to ON.
Any unique prefix of a valid value may be used to set the value of this variable.
This variable was added in MySQL 4.0.3.
Normally, when one client acquires a WRITE
lock on a MyISAM table, other clients are
not blocked from issuing statements that read from the table
if the query results are present in the query cache. Setting
this variable to 1 causes acquisition of a
WRITE lock for a table to invalidate any
queries in the query cache that refer to the table. This
forces other clients that attempt to access the table to wait
while the lock is in effect. This variable was added in MySQL
4.0.19.
The size of the persistent buffer used for statement parsing
and execution. This buffer is not freed between statements. If
you are running complex queries, a larger
query_prealloc_size value
might be helpful in improving performance, because it can
reduce the need for the server to perform memory allocation
during query execution operations.
This variable was added in MySQL 4.0.16.
The rand_seed1 and
rand_seed2 variables exist as
session variables only, and can be set but not read. They are
not shown in the output of SHOW
VARIABLES. These two variables were added in MySQL
4.0.5.
The purpose of these variables is to support replication of
the RAND() function. For
statements that invoke RAND(),
the master passes two values to the slave, where they are used
to seed the random number generator. The slave uses these
values to set the session variables
rand_seed1 and
rand_seed2 so that
RAND() on the slave generates
the same value as on the master.
See the description for
rand_seed1.
The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.
Each thread that does a sequential scan allocates a buffer of this size (in bytes) for each table it scans. If you do many sequential scans, you might want to increase this value.
read_buffer_size and
read_rnd_buffer_size are not
specific to any storage engine and apply in a general manner
for optimization. See Section 7.8.4, “How MySQL Uses Memory”, for
example.
Before MySQL 4.0.3, this variable was named
record_buffer.
This variable is off by default. When it is enabled, the
server permits no updates except from users that have the
SUPER privilege or (on a slave
server) from updates performed by slave threads. On a slave
server, this can be useful to ensure that the slave accepts
updates only from its master server and not from clients.
read_only exists only as a
GLOBAL variable, so changes to its value
require the SUPER privilege.
Changes to read_only on a
master server are not replicated to slave servers. The value
can be set on a slave server independent of the setting on the
master.
This variable was added in MySQL 4.0.14.
When reading rows in sorted order following a key-sorting
operation, the rows are read through this buffer to avoid disk
seeks. See Section 7.3.1.7, “ORDER BY Optimization”. Setting
the variable to a large value can improve ORDER
BY performance by a lot. However, this is a buffer
allocated for each client, so you should not set the global
variable to a large value. Instead, change the session
variable only from within those clients that need to run large
queries.
read_buffer_size and
read_rnd_buffer_size are not
specific to any storage engine and apply in a general manner
for optimization. See Section 7.8.4, “How MySQL Uses Memory”, for
example.
Before MySQL 4.0.3, this variable was named
record_rnd_buffer.
Disables or enables automatic purging of relay log files as
soon as they are not needed any more. The default value is 1
(ON).
The maximum amount of space to use for all relay logs.
Do not show databases for which the user has no database or
table privileges. This can improve security if you are
concerned about people being able to see what databases other
users have. See also
skip_show_database.
This variable was removed in MySQL 4.0.5. Beginning with this
version, you should instead use the SHOW
DATABASES privilege to control access by MySQL
accounts to databases.
If the MySQL server has been started with the
--secure-auth option, it blocks
connections from all accounts that have passwords stored in
the old (pre-4.1) format. In that case, the value of this
variable is ON, otherwise it is
OFF.
You should enable this option if you want to prevent all use of passwords in the old format (and hence insecure communication over the network). This variable was added in MySQL 4.1.1.
Server startup fails with an error if this option is enabled and the privilege tables are in pre-4.1 format.
The server ID, used in replication to give each master and
slave a unique identity. This variable is set by the
--server-id option. For each
server participating in replication, you should pick a
positive integer in the range from 1 to
232 – 1 to act as that
server's ID.
(Windows only.) Whether the server permits shared-memory connections. This variable was added in MySQL 4.1.1.
(Windows only.) The name of shared memory to use for shared-memory connections. This is useful when running multiple MySQL instances on a single physical machine. This variable was added in MySQL 4.1.0.
This is OFF if mysqld
uses external locking, ON if external
locking is disabled. Before MySQL 4.0.3, this variable was
named skip_locking.
This is ON if the server permits only local
(non-TCP/IP) connections. On Unix, local connections use a
Unix socket file. On Windows, local connections use a named
pipe or shared memory. On NetWare, only TCP/IP connections are
supported, so do not set this variable to
ON. This variable can be set to
ON with the
--skip-networking option. This
variable was added in MySQL 3.22.23.
This prevents people from using the SHOW
DATABASES statement if they do not have the
SHOW DATABASES privilege. This
can improve security if you are concerned about people being
able to see what databases other users have. See also
safe_show_database. This
variable was added in MySQL 3.23.4. As of MySQL 4.0.2, its
effect also depends on the SHOW
DATABASES privilege: If the variable value is
ON, the SHOW
DATABASES statement is permitted only to users who
have the SHOW DATABASES
privilege, and the statement displays all database names. If
the value is OFF, SHOW
DATABASES is permitted to all users, but displays
each database name only if the user has the
SHOW DATABASES privilege or
some privilege for the database. (Note that
any global privilege is considered a
privilege for the database.)
If creating a thread takes longer than this many seconds, the
server increments the
Slow_launch_threads status
variable. This variable was added in MySQL 3.23.15.
On Unix platforms, this variable is the name of the socket
file that is used for local client connections. The default is
/tmp/mysql.sock. (For some distribution
formats, the directory might be different, such as
/var/lib/mysql for RPMs.)
On Windows, this variable is the name of the named pipe that
is used for local client connections. The default value is
MySQL (not case sensitive).
Each session that needs to do a sort allocates a buffer of
this size. sort_buffer_size
is not specific to any storage engine and applies in a general
manner for optimization. See
Section 7.3.1.7, “ORDER BY Optimization”, for example.
If you see many
Sort_merge_passes per second
in SHOW GLOBAL
STATUS output, you can consider increasing the
sort_buffer_size value to
speed up ORDER BY or GROUP
BY operations that cannot be improved with query
optimization or improved indexing. The entire buffer is
allocated even if it is not all needed, so setting it larger
than required globally will slow down most queries that sort.
It is best to increase it as a session setting, and only for
the sessions that need a larger size. On Linux, there are
thresholds of 256KB and 2MB where larger values may
significantly slow down memory allocation, so you should
consider staying below one of those values. Experiment to find
the best value for your workload. See
Section B.5.4.4, “Where MySQL Stores Temporary Files”.
| System Variable | Name | sql_auto_is_null | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 1 | ||
If this variable is set to 1 (the default), then after a
statement that successfully inserts an automatically generated
AUTO_INCREMENT value, you can find that
value by issuing a statement of the following form:
SELECT * FROMtbl_nameWHEREauto_colIS NULL
If the statement returns a row, the value returned is the same
as if you invoked the
LAST_INSERT_ID() function. For
details, including the return value after a multiple-row
insert, see Section 11.13, “Information Functions”. If no
AUTO_INCREMENT value was successfully
inserted, the SELECT statement
returns no row.
The behavior of retrieving an
AUTO_INCREMENT value by using an
IS NULL comparison is used by
some ODBC programs, such as Access. See
Obtaining Auto-Increment Values.
This behavior can be disabled by setting
sql_auto_is_null to 0.
| System Variable | Name | sql_big_selects | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 1 | ||
If set to 0, MySQL aborts
SELECT statements that are
likely to take a very long time to execute (that is,
statements for which the optimizer estimates that the number
of examined rows exceeds the value of
max_join_size). This is
useful when an inadvisable WHERE statement
has been issued. The default value for a new connection is 1,
which permits all SELECT
statements.
If you set the max_join_size
system variable to a value other than
DEFAULT,
sql_big_selects is set to 0.
| System Variable | Name | sql_buffer_result | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 0 | ||
If set to 1,
sql_buffer_result forces
results from SELECT statements
to be put into temporary tables. This helps MySQL free the
table locks early and can be beneficial in cases where it
takes a long time to send results to the client. The default
value is 0. This variable was added in MySQL 3.23.13.
| System Variable | Name | sql_log_bin | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
If set to 0, no logging is done to the binary log for the
client. The client must have the
SUPER privilege to set this
option. The default value is 1. This variable was added in
MySQL 3.23.16.
| System Variable | Name | sql_log_off | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 0 | ||
If set to 1, no logging is done to the general query log for
this client. The client must have the
SUPER privilege to set this
option. The default value is 0.
| System Variable | Name | sql_log_update | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
If set to 0, no logging is done to the
update log for the client. The client must have the
SUPER privilege to set this
option. The default value is 1. This variable was added in
MySQL 3.22.5.
The current server SQL mode. This variable was added in MySQL 3.23.41. It can be set dynamically as of MySQL 4.1.1. See Section 5.1.6, “Server SQL Modes”.
If set to 1 (the default), warnings of Note
level are recorded. If set to 0, Note
warnings are suppressed. mysqldump includes
output to set this variable to 0 so that reloading the dump
file does not produce warnings for events that do not affect
the integrity of the reload operation.
sql_notes was added in MySQL
4.1.11.
If set to 1 (the default), the server quotes identifiers for
SHOW CREATE TABLE and
SHOW CREATE DATABASE
statements. If set to 0, quoting is disabled. This option is
enabled by default so that replication works for identifiers
that require quoting. See Section 12.4.5.7, “SHOW CREATE TABLE Syntax”,
and Section 12.4.5.6, “SHOW CREATE DATABASE Syntax”. This variable was
added in MySQL 3.23.26.
If set to 1, MySQL aborts
UPDATE or
DELETE statements that do not
use a key in the WHERE clause or a
LIMIT clause. This makes it possible to
catch UPDATE or
DELETE statements where keys
are not used properly and that would probably change or delete
a large number of rows. The default value is 0. This variable
was added in MySQL 3.22.32.
The maximum number of rows to return from
SELECT statements. The default
value for a new connection is the maximum number of rows that
the server permits per table, which depends on the server
configuration and may be affected if the server build was
configured with
--with-big-tables. Typical
default values are (232)–1 or
(264)–1. If you have changed
the limit, the default value can be restored by assigning a
value of DEFAULT.
If a SELECT has a
LIMIT clause, the LIMIT
takes precedence over the value of
sql_select_limit.
sql_select_limit does not
apply to SELECT statements
executed within stored routines. It also does not apply to
SELECT statements that do not
produce a result set to be returned to the client. These
include SELECT statements in
subqueries,
CREATE TABLE ...
SELECT, and
INSERT INTO ...
SELECT.
This variable controls whether single-row
INSERT statements produce an
information string if warnings occur. The default is 0. Set
the value to 1 to produce an information string. This variable
was added in MySQL 3.22.11.
This variable is a synonym for
table_type. It was added in
MySQL 4.1.2.
If this variable is set to 1, when any nontemporary table is
created its .frm file is synchronized to
disk (using fdatasync()). This is slower
but safer in case of a crash. The default is 1. This was added
as a command-line option in MySQL 4.0.18. It is also a
settable global variable as of MySQL 4.1.3.
The server system time zone. When the server begins executing,
it inherits a time zone setting from the machine defaults,
possibly modified by the environment of the account used for
running the server or the startup script. The value is used to
set system_time_zone.
Typically the time zone is specified by the
TZ environment variable. It also can be
specified using the
--timezone option of the
mysqld_safe script.
The system_time_zone variable
differs from time_zone.
Although they might have the same value, the latter variable
is used to initialize the time zone for each client that
connects. See Section 9.7, “MySQL Server Time Zone Support”.
system_time_zone was added in
MySQL 4.1.3.
The number of open tables for all threads. Increasing this
value increases the number of file descriptors that
mysqld requires. You can check whether you
need to increase the table cache by checking the
Opened_tables status
variable. See Section 5.1.5, “Server Status Variables”. If
the value of Opened_tables
is large and you do not do
FLUSH TABLES
often (which just forces all tables to be closed and
reopened), then you should increase the value of the
table_cache variable. For
more information about the table cache, see
Section 7.7.2, “How MySQL Opens and Closes Tables”.
The default table type (storage engine). To set the table type
at server startup, use the
--default-table-type option.
This variable was added in MySQL 3.23.0. See
Section 5.1.2, “Server Command Options”.
How many threads the server should cache for reuse. When a
client disconnects, the client's threads are put in the cache
if there are fewer than
thread_cache_size threads
there. Requests for threads are satisfied by reusing threads
taken from the cache if possible, and only when the cache is
empty is a new thread created. This variable can be increased
to improve performance if you have a lot of new connections.
Normally, this does not provide a notable performance
improvement if you have a good thread implementation. However,
if your server sees hundreds of connections per second you
should normally set
thread_cache_size high enough
so that most new connections use cached threads. By examining
the difference between the
Connections and
Threads_created status
variables, you can see how efficient the thread cache is. For
details, see Section 5.1.5, “Server Status Variables”. This
variable was added in MySQL 3.23.16.
This variable is specific to Solaris systems, for which
mysqld invokes the
thr_setconcurrency() with the variable
value. This function enables applications to give the threads
system a hint about the desired number of threads that should
be run at the same time. This variable was added in MySQL
3.23.7.
The stack size for each thread. Many of the limits detected by
the crash-me test are dependent on this
value. The default is large enough for normal operation. See
Section 7.1.3, “The MySQL Benchmark Suite”. The default is 64KB before
MySQL 4.0.10 and 192KB thereafter. If the thread stack size is
too small, it limits the complexity of the SQL statements that
the server can handle, the recursion depth of stored
procedures, and other memory-consuming actions.
This variable is unused.
The current time zone. This variable is used to initialize the
time zone for each client that connects. By default, the
initial value of this is 'SYSTEM' (which
means, “use the value of
system_time_zone”).
The value can be specified explicitly at server startup with
the --default-time-zone option.
See Section 9.7, “MySQL Server Time Zone Support”. This variable was
added in MySQL 4.1.3.
timestamp =
{
timestamp_value |
DEFAULT}
Set the time for this client. This is used to get the original
timestamp if you use the binary log to restore rows.
timestamp_value should be a Unix
epoch timestamp, not a MySQL timestamp.
The time zone for the server. This is set from the
TZ environment variable when
mysqld is started. The time zone also can
be set by giving a
--timezone argument to
mysqld_safe. This variable was added in
MySQL 3.23.15. As of MySQL 4.1.3, it is obsolete and has been
replaced by the
system_time_zone variable.
See Section B.5.4.6, “Time Zone Problems”.
The maximum size of internal in-memory temporary tables. (The
actual limit is determined as the minimum of
tmp_table_size and
max_heap_table_size.) If an
in-memory temporary table exceeds the limit, MySQL
automatically converts it to an on-disk
MyISAM table. Increase the value of
tmp_table_size (and
max_heap_table_size if
necessary) if you do many advanced GROUP BY
queries and you have lots of memory. This variable does not
apply to user-created MEMORY tables.
You can compare the number of internal on-disk temporary
tables created to the total number of internal temporary
tables created by comparing the values of the
Created_tmp_disk_tables and
Created_tmp_tables
variables.
See also Section 7.7.4, “How MySQL Uses Internal Temporary Tables”.
The directory used for temporary files and temporary tables.
Starting from MySQL 4.1, this variable can be set to a list of
several paths that are used in round-robin fashion. Paths
should be separated by colon characters
(“:”) on Unix and semicolon
characters (“;”) on Windows,
NetWare, and OS/2.
The multiple-directory feature can be used to spread the load
between several physical disks. If the MySQL server is acting
as a replication slave, you should not set
tmpdir to point to a
directory on a memory-based file system or to a directory that
is cleared when the server host restarts. A replication slave
needs some of its temporary files to survive a machine restart
so that it can replicate temporary tables or
LOAD DATA
INFILE operations. If files in the temporary file
directory are lost when the server restarts, replication
fails. However, if you are using MySQL 4.0.0 or later, you can
set the slave's temporary directory using the
slave_load_tmpdir variable.
In that case, the slave will not use the general
tmpdir value and you can set
tmpdir to a nonpermanent
location.
This variable was added in MySQL 3.22.4.
The amount in bytes by which to increase a per-transaction
memory pool which needs memory. See the description of
transaction_prealloc_size.
This variable was added in MySQL 4.0.16.
There is a per-transaction memory pool from which various
transaction-related allocations take memory. The initial size
of the pool in bytes is
transaction_prealloc_size.
For every allocation that cannot be satisfied from the pool
because it has insufficient memory available, the pool is
increased by
transaction_alloc_block_size
bytes. When the transaction ends, the pool is truncated to
transaction_prealloc_size
bytes.
By making
transaction_prealloc_size
sufficiently large to contain all statements within a single
transaction, you can avoid many malloc()
calls. This variable was added in MySQL 4.0.16.
The default transaction isolation level. This variable was added in MySQL 4.0.3.
This variable is set by the
SET
TRANSACTION ISOLATION LEVEL statement. See
Section 12.3.6, “SET TRANSACTION Syntax”. If you set
tx_isolation directly to an
isolation level name that contains a space, the name should be
enclosed within quotation marks, with the space replaced by a
dash. For example:
SET tx_isolation = 'READ-COMMITTED';
Any unique prefix of a valid value may be used to set the value of this variable.
| System Variable | Name | unique_checks | |
| Variable Scope | Session | ||
| Dynamic Variable | Yes | ||
| Permitted Values | Type | boolean | |
| Default | 1 | ||
If set to 1 (the default), uniqueness checks for secondary
indexes in InnoDB tables are performed. If
set to 0, storage engines are permitted to assume that
duplicate keys are not present in input data. If you know for
certain that your data does not contain uniqueness violations,
you can set this to 0 to speed up large table imports to
InnoDB. This variable was added in MySQL
3.23.52.
Note that setting this variable to 0 does not require storage engines to ignore duplicate keys. An engine is still permitted to check for them and issue duplicate-key errors if it detects them.
The version number for the server.
The BDB storage engine version. This
variable was added in MySQL 3.23.31 with the name
bdb_version and renamed to
version_bdb in MySQL 4.1.1.
| System Variable | Name | version_comment | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The configure script has a
--with-comment option that permits a comment
to be specified when building MySQL. This variable contains
the value of that comment. This variable was added in MySQL
4.0.17.
| System Variable | Name | version_compile_machine | |
| Variable Scope | Global | ||
| Dynamic Variable | No | ||
| Permitted Values | Type | string | |
The type of machine or architecture on which MySQL was built. This variable was added in MySQL 4.1.1.
The type of operating system on which MySQL was built. This variable was added in MySQL 4.0.19.
The number of seconds the server waits for activity on a noninteractive connection before closing it. This timeout applies only to TCP/IP and Unix socket file connections, not to connections made using named pipes, or shared memory.
On thread startup, the session
wait_timeout value is
initialized from the global
wait_timeout value or from
the global
interactive_timeout value,
depending on the type of client (as defined by the
CLIENT_INTERACTIVE connect option to
mysql_real_connect()). See
also interactive_timeout.
The number of errors, warnings, and notes that resulted from the last statement that generated messages. This variable is read only. See Section 12.4.5.26, “SHOW WARNINGS Syntax”.
This variable was added in MySQL 4.1.0.
The MySQL server maintains many system variables that indicate how
it is configured. Section 5.1.3, “Server System Variables”,
describes the meaning of these variables. Each system variable has
a default value. System variables can be set at server startup
using options on the command line or in an option file. As of
MySQL 4.0.3, most of them can be changed dynamically while the
server is running by means of the
SET
statement, which enables you to modify operation of the server
without having to stop and restart it. You can refer to system
variable values in expressions.
Beginning with MySQL 4.0.3, the server maintains two kinds of system variables. Global variables affect the overall operation of the server. Session variables affect its operation for individual client connections. A given system variable can have both a global and a session value. Global and session system variables are related as follows:
When the server starts, it initializes all global variables to their default values. These defaults can be changed by options specified on the command line or in an option file. (See Section 4.2.3, “Specifying Program Options”.)
The server also maintains a set of session variables for each
client that connects. The client's session variables are
initialized at connect time using the current values of the
corresponding global variables. For example, the client's SQL
mode is controlled by the session
sql_mode value, which is
initialized when the client connects to the value of the
global sql_mode value.
System variable values can be set globally at server startup by
using options on the command line or in an option file. When you
use a startup option to set a variable that takes a numeric value,
the value can be given with a suffix of K,
M, or G (either uppercase or
lowercase) to indicate a multiplier of 1024,
10242 or
10243; that is, units of kilobytes,
megabytes, or gigabytes, respectively. Thus, the following command
starts the server with a query cache size of 16 megabytes and a
maximum packet size of one gigabyte:
mysqld --query_cache_size=16M --max_allowed_packet=1G
Before MySQL 4.0.3, use this syntax instead:
mysqld --set-variable=query_cache_size=16M \
--set-variable=max_allowed_packet=1G
Within an option file, those variables are set like this:
[mysqld] query_cache_size=16M max_allowed_packet=1G
Or like this before MySQL 4.0.2:
[mysqld] set-variable=query_cache_size=16M set-variable=max_allowed_packet=1G
The lettercase of suffix letters does not matter;
16M and 16m are equivalent,
as are 1G and 1g.
If you want to restrict the maximum value to which a system
variable can be set at runtime with the
SET
statement, you can specify this maximum by using an option of the
form
--maximum-
at server startup. For example, to prevent the value of
var_name=valuequery_cache_size from being
increased to more than 32MB at runtime, use the option
--maximum-query_cache_size=32M. This feature is
available as of MySQL 4.0.2.
Many system variables are dynamic and can be changed while the
server runs by using the
SET
statement. For a list, see
Section 5.1.4.2, “Dynamic System Variables”. To change a system
variable with
SET, refer
to it as var_name, optionally preceded
by a modifier:
To indicate explicitly that a variable is a global variable,
precede its name by GLOBAL or
@@global.. The
SUPER privilege is required to
set global variables.
To indicate explicitly that a variable is a session variable,
precede its name by SESSION,
@@session., or @@.
Setting a session variable requires no special privilege, but
a client can change only its own session variables, not those
of any other client.
LOCAL and @@local. are
synonyms for SESSION and
@@session..
If no modifier is present,
SET
changes the session variable.
A SET
statement can contain multiple variable assignments, separated by
commas. If you set several system variables, the most recent
GLOBAL or SESSION modifier
in the statement is used for following variables that have no
modifier specified.
Examples:
SET sort_buffer_size=10000; SET @@local.sort_buffer_size=10000; SET GLOBAL sort_buffer_size=1000000, SESSION sort_buffer_size=1000000; SET @@sort_buffer_size=1000000; SET @@global.sort_buffer_size=1000000, @@local.sort_buffer_size=1000000;
The @@
syntax for system variables is supported for compatibility with
some other database systems.
var_name
If you change a session system variable, the value remains in effect until your session ends or until you change the variable to a different value. The change is not visible to other clients.
If you change a global system variable, the value is remembered
and used for new connections until the server restarts. (To make a
global system variable setting permanent, you should set it in an
option file.) The change is visible to any client that accesses
that global variable. However, the change affects the
corresponding session variable only for clients that connect after
the change. The global variable change does not affect the session
variable for any client that is currently connected (not even that
of the client that issues the
SET GLOBAL
statement).
To prevent incorrect usage, MySQL produces an error if you use
SET GLOBAL
with a variable that can only be used with
SET SESSION
or if you do not specify GLOBAL (or
@@global.) when setting a global variable.
To set a SESSION variable to the
GLOBAL value or a GLOBAL
value to the compiled-in MySQL default value, use the
DEFAULT keyword. For example, the following two
statements are identical in setting the session value of
max_join_size to the global
value:
SET max_join_size=DEFAULT; SET @@session.max_join_size=@@global.max_join_size;
Not all system variables can be set to DEFAULT.
In such cases, use of DEFAULT results in an
error.
You can refer to the values of specific global or sesson system
variables in expressions by using one of the
@@-modifiers. For example, you can retrieve
values in a SELECT statement like
this:
SELECT @@global.sql_mode, @@session.sql_mode, @@sql_mode;
When you refer to a system variable in an expression as
@@ (that is,
when you do not specify var_name@@global. or
@@session.), MySQL returns the session value if
it exists and the global value otherwise. (This differs from
SET @@, which always refers to
the session value.)
var_name =
value
Some variables displayed by SHOW VARIABLES
may not be available using SELECT
@@ syntax; an
var_nameUnknown system variable occurs. As a
workaround in such cases, you can use SHOW VARIABLES
LIKE '.
var_name'
Suffixes for specifying a value multiplier can be used when
setting a variable at server startup, but not to set the value
with SET at
runtime. On the other hand, with
SET you can
assign a variable's value using an expression, which is not true
when you set a variable at server startup. For example, the first
of the following lines is legal at server startup, but the second
is not:
shell>mysql --max_allowed_packet=16Mshell>mysql --max_allowed_packet=16*1024*1024
Conversely, the second of the following lines is legal at runtime, but the first is not:
mysql>SET GLOBAL max_allowed_packet=16M;mysql>SET GLOBAL max_allowed_packet=16*1024*1024;
Some system variables can be enabled with the
SET
statement by setting them to ON or
1, or disabled by setting them to
OFF or 0. However, to set
such a variable on the command line or in an option file, you
must set it to 1 or 0;
setting it to ON or OFF
will not work. For example, on the command line,
--delay_key_write=1 works but
--delay_key_write=ON does not.
To display system variable names and values, use the
SHOW VARIABLES statement:
mysql> SHOW VARIABLES;
+---------------------------------+-------------------------------------+
| Variable_name | Value |
+---------------------------------+-------------------------------------+
| back_log | 50 |
| basedir | /usr/local/mysql |
| bdb_cache_size | 8388600 |
| bdb_home | /usr/local/mysql |
| bdb_log_buffer_size | 32768 |
| bdb_logdir | |
| bdb_max_lock | 10000 |
| bdb_shared_data | OFF |
| bdb_tmpdir | /tmp/ |
| binlog_cache_size | 32768 |
| bulk_insert_buffer_size | 8388608 |
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_database | latin1 |
| character_set_results | latin1 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/local/mysql/share/charsets/ |
| collation_connection | latin1_swedish_ci |
| collation_database | latin1_swedish_ci |
| collation_server | latin1_swedish_ci |
...
| innodb_additional_mem_pool_size | 1048576 |
| innodb_autoextend_increment | 8 |
| innodb_buffer_pool_awe_mem_mb | 0 |
| innodb_buffer_pool_size | 8388608 |
| innodb_data_file_path | ibdata1:10M:autoextend |
| innodb_data_home_dir | |
...
| version | 4.1.18-max-log |
| version_comment | MySQL Community Edition - Max (GPL) |
| version_compile_machine | i686 |
| version_compile_os | pc-linux-gnu |
| wait_timeout | 28800 |
+---------------------------------+-------------------------------------+
With a LIKE clause, the statement
displays only those variables that match the pattern. To obtain a
specific variable name, use a LIKE
clause as shown:
SHOW VARIABLES LIKE 'max_join_size'; SHOW SESSION VARIABLES LIKE 'max_join_size';
To get a list of variables whose name match a pattern, use the
“%” wildcard character in a
LIKE clause:
SHOW VARIABLES LIKE '%size%'; SHOW GLOBAL VARIABLES LIKE '%size%';
Wildcard characters can be used in any position within the pattern
to be matched. Strictly speaking, because
“_” is a wildcard that matches any
single character, you should escape it as
“\_” to match it literally. In
practice, this is rarely necessary.
For SHOW VARIABLES, if you specify
neither GLOBAL nor SESSION,
MySQL returns SESSION values.
The reason for requiring the GLOBAL keyword
when setting GLOBAL-only variables but not when
retrieving them is to prevent problems in the future. If we were
to remove a SESSION variable that has the same
name as a GLOBAL variable, a client with the
SUPER privilege might accidentally
change the GLOBAL variable rather than just the
SESSION variable for its own connection. If we
add a SESSION variable with the same name as a
GLOBAL variable, a client that intends to
change the GLOBAL variable might find only its
own SESSION variable changed.
Structured system variables are supported beginning with MySQL 4.1.1. A structured variable differs from a regular system variable in two respects:
Its value is a structure with components that specify server parameters considered to be closely related.
There might be several instances of a given type of structured variable. Each one has a different name and refers to a different resource maintained by the server.
In MySQL 4.1 (4.1.1 and above), MySQL supports one structured variable type. It specifies parameters that govern the operation of key caches. A key cache structured variable has these components:
The purpose of this section is to describe the syntax for referring to structured variables. Key cache variables are used for syntax examples, but specific details about how key caches operate are found elsewhere, in Section 7.5.1, “The MyISAM Key Cache”.
To refer to a component of a structured variable instance, you
can use a compound name in
instance_name.component_name format.
Examples:
hot_cache.key_buffer_size hot_cache.key_cache_block_size cold_cache.key_cache_block_size
For each structured system variable, an instance with the name
of default is always predefined. If you refer
to a component of a structured variable without any instance
name, the default instance is used. Thus,
default.key_buffer_size and
key_buffer_size both refer to
the same system variable.
Structured variable instances and components follow these naming rules:
For a given type of structured variable, each instance must
have a name that is unique within
variables of that type. However, instance names need not be
unique across structured variable
types. For example, each structured variable has an instance
named default, so
default is not unique across variable
types.
The names of the components of each structured variable type must be unique across all system variable names. If this were not true (that is, if two different types of structured variables could share component member names), it would not be clear which default structured variable to use for references to member names that are not qualified by an instance name.
If a structured variable instance name is not legal as an
unquoted identifier, refer to it as a quoted identifier
using backticks. For example, hot-cache
is not legal, but `hot-cache` is.
global, session, and
local are not legal instance names. This
avoids a conflict with notation such as
@@global.
for referring to nonstructured system variables.
var_name
At the moment, the first two rules have no possibility of being violated because the only structured variable type is the one for key caches. These rules will assume greater significance if some other type of structured variable is created in the future.
With one exception, it is permissible to refer to structured variable components using compound names in any context where simple variable names can occur. For example, you can assign a value to a structured variable using a command-line option:
shell> mysqld --hot_cache.key_buffer_size=64K
In an option file, use this syntax:
[mysqld] hot_cache.key_buffer_size=64K
If you start the server with such an option, it creates a key
cache named hot_cache with a size of 64KB in
addition to the default key cache that has a default size of
8MB.
Suppose that you start the server as follows:
shell>mysqld --key_buffer_size=256K \--extra_cache.key_buffer_size=128K \--extra_cache.key_cache_block_size=2048
In this case, the server sets the size of the default key cache
to 256KB. (You could also have written
--default.key_buffer_size=256K.) In addition,
the server creates a second key cache named
extra_cache that has a size of 128KB, with
the size of block buffers for caching table index blocks set to
2048 bytes.
The following example starts the server with three different key caches having sizes in a 3:1:1 ratio:
shell>mysqld --key_buffer_size=6M \--hot_cache.key_buffer_size=2M \--cold_cache.key_buffer_size=2M
Structured variable values may be set and retrieved at runtime
as well. For example, to set a key cache named
hot_cache to a size of 10MB, use either of
these statements:
mysql>SET GLOBAL hot_cache.key_buffer_size = 10*1024*1024;mysql>SET @@global.hot_cache.key_buffer_size = 10*1024*1024;
To retrieve the cache size, do this:
mysql> SELECT @@global.hot_cache.key_buffer_size;
However, the following statement does not work. The variable is
not interpreted as a compound name, but as a simple string for a
LIKE pattern-matching operation:
mysql> SHOW GLOBAL VARIABLES LIKE 'hot_cache.key_buffer_size';
This is the exception to being able to use structured variable names anywhere a simple variable name may occur.
Beginning with MySQL 4.0.3, many server system variables are
dynamic and can be set at runtime using
SET GLOBAL
or SET
SESSION. You can also select their values using
SELECT. See
Section 5.1.4, “Using System Variables”.
The following table shows the full list of all dynamic system
variables. The last column indicates for each variable whether
GLOBAL or SESSION (or
both) apply. The table also lists session options that can be
set with the
SET
statement. Section 5.1.3, “Server System Variables”, discusses
these options.
Variables that have a type of “string” take a
string value. Variables that have a type of
“numeric” take a numeric value. Variables that have
a type of “boolean” can be set to 0, 1,
ON or OFF. (If you set
them on the command line or in an option file, use the numeric
values.) Variables that are marked as “enumeration”
normally should be set to one of the available values for the
variable, but can also be set to the number that corresponds to
the desired enumeration value. For enumerated system variables,
the first enumeration value corresponds to 0. This differs from
ENUM columns, for which the first
enumeration value corresponds to 1.
Table 5.3 Dynamic Variable Summary
| Variable Name | Variable Type | Variable Scope |
|---|---|---|
autocommit | boolean | SESSION |
big_tables | boolean | SESSION |
binlog_cache_size | numeric | GLOBAL |
bulk_insert_buffer_size | numeric | GLOBAL | SESSION |
character_set_client | string | GLOBAL | SESSION |
character_set_connection | string | GLOBAL | SESSION |
character_set_database | string | GLOBAL | SESSION |
character_set_results | string | GLOBAL | SESSION |
character_set_server | string | GLOBAL | SESSION |
collation_connection | string | GLOBAL | SESSION |
collation_database | string | GLOBAL | SESSION |
collation_server | string | GLOBAL | SESSION |
concurrent_insert | boolean | GLOBAL |
connect_timeout | numeric | GLOBAL |
debug | string | GLOBAL | SESSION |
storage_engine | enumeration | GLOBAL | SESSION |
default_week_format | numeric | GLOBAL | SESSION |
delay_key_write | enumeration | GLOBAL |
delayed_insert_limit | numeric | GLOBAL |
delayed_insert_timeout | numeric | GLOBAL |
delayed_queue_size | numeric | GLOBAL |
expire_logs_days | numeric | GLOBAL |
flush | boolean | GLOBAL |
flush_time | numeric | GLOBAL |
foreign_key_checks | boolean | SESSION |
ft_boolean_syntax | string | GLOBAL |
group_concat_max_len | numeric | GLOBAL | SESSION |
identity | numeric | SESSION |
init_connect | string | GLOBAL |
init_slave | string | GLOBAL |
innodb_autoextend_increment | numeric | GLOBAL |
innodb_fast_shutdown | numeric | GLOBAL |
innodb_flush_log_at_trx_commit | enumeration | GLOBAL |
innodb_max_dirty_pages_pct | numeric | GLOBAL |
innodb_max_purge_lag | numeric | GLOBAL |
innodb_table_locks | boolean | GLOBAL | SESSION |
innodb_thread_concurrency | numeric | GLOBAL |
insert_id | numeric | SESSION |
interactive_timeout | numeric | GLOBAL | SESSION |
join_buffer_size | numeric | GLOBAL | SESSION |
key_buffer_size | numeric | GLOBAL |
key_cache_age_threshold | numeric | GLOBAL |
key_cache_block_size | numeric | GLOBAL |
key_cache_division_limit | numeric | GLOBAL |
last_insert_id | numeric | SESSION |
lc_time_names | string | GLOBAL | SESSION |
local_infile | boolean | GLOBAL |
log_warnings | numeric | GLOBAL | SESSION |
long_query_time | numeric | GLOBAL | SESSION |
low_priority_updates | boolean | GLOBAL | SESSION |
max_allowed_packet | numeric | GLOBAL |
max_binlog_cache_size | numeric | GLOBAL |
max_binlog_size | numeric | GLOBAL |
max_connect_errors | numeric | GLOBAL |
max_connections | numeric | GLOBAL |
max_delayed_threads | numeric | GLOBAL | SESSION |
max_error_count | numeric | GLOBAL | SESSION |
max_heap_table_size | numeric | GLOBAL | SESSION |
max_insert_delayed_threads | numeric | GLOBAL | SESSION |
max_join_size | numeric | GLOBAL | SESSION |
max_length_for_sort_data | numeric | GLOBAL | SESSION |
max_prepared_stmt_count | numeric | GLOBAL |
max_relay_log_size | numeric | GLOBAL |
max_seeks_for_key | numeric | GLOBAL | SESSION |
max_sort_length | numeric | GLOBAL | SESSION |
max_user_connections | numeric | GLOBAL |
max_write_lock_count | numeric | GLOBAL |
myisam_data_pointer_size | numeric | GLOBAL |
myisam_max_sort_file_size | numeric | GLOBAL |
myisam_repair_threads | numeric | GLOBAL | SESSION |
myisam_sort_buffer_size | numeric | GLOBAL | SESSION |
myisam_stats_method | enumeration | GLOBAL | SESSION |
ndb_autoincrement_prefetch_sz | numeric | GLOBAL | SESSION |
ndb_cache_check_time | numeric | GLOBAL |
ndb_force_send | boolean | GLOBAL | SESSION |
ndb_index_stat_cache_entries | numeric | GLOBAL | SESSION |
ndb_index_stat_enable | boolean | GLOBAL | SESSION |
ndb_index_stat_update_freq | numeric | GLOBAL | SESSION |
ndb_use_exact_count | boolean | GLOBAL | SESSION |
ndb_use_transactions | boolean | GLOBAL | SESSION |
net_buffer_length | numeric | GLOBAL | SESSION |
net_read_timeout | numeric | GLOBAL | SESSION |
net_retry_count | numeric | GLOBAL | SESSION |
net_write_timeout | numeric | GLOBAL | SESSION |
new | boolean | GLOBAL | SESSION |
old_passwords | boolean | GLOBAL | SESSION |
preload_buffer_size | numeric | GLOBAL | SESSION |
pseudo_thread_id | numeric | SESSION |
query_alloc_block_size | numeric | GLOBAL | SESSION |
query_cache_limit | numeric | GLOBAL |
query_cache_min_res_unit | numeric | GLOBAL |
query_cache_size | numeric | GLOBAL |
query_cache_type | enumeration | GLOBAL | SESSION |
query_cache_wlock_invalidate | boolean | GLOBAL | SESSION |
query_prealloc_size | numeric | GLOBAL | SESSION |
rand_seed1 | numeric | SESSION |
rand_seed2 | numeric | SESSION |
range_alloc_block_size | numeric | GLOBAL | SESSION |
read_buffer_size | numeric | GLOBAL | SESSION |
read_only | boolean | GLOBAL |
read_rnd_buffer_size | numeric | GLOBAL | SESSION |
relay_log_purge | boolean | GLOBAL |
rpl_recovery_rank | numeric | GLOBAL |
safe_show_database | boolean | GLOBAL |
secure_auth | boolean | GLOBAL |
server_id | numeric | GLOBAL |
slave_compressed_protocol | boolean | GLOBAL |
slave_net_timeout | numeric | GLOBAL |
slave_transaction_retries | numeric | GLOBAL |
slow_launch_time | numeric | GLOBAL |
sort_buffer_size | numeric | GLOBAL | SESSION |
sql_auto_is_null | boolean | SESSION |
sql_big_selects | boolean | SESSION |
sql_big_tables | boolean | SESSION |
sql_buffer_result | boolean | SESSION |
sql_log_bin | boolean | SESSION |
sql_log_off | boolean | SESSION |
sql_log_update | boolean | SESSION |
sql_low_priority_updates | boolean | GLOBAL | SESSION |
sql_max_join_size | numeric | GLOBAL | SESSION |
sql_mode | set | GLOBAL | SESSION |
sql_notes | boolean | SESSION |
sql_quote_show_create | boolean | SESSION |
sql_safe_updates | boolean | SESSION |
sql_select_limit | numeric | GLOBAL | SESSION |
sql_slave_skip_counter | numeric | GLOBAL |
sql_warnings | boolean | SESSION |
storage_engine | enumeration | GLOBAL | SESSION |
sync_binlog | numeric | GLOBAL |
sync_frm | boolean | GLOBAL |
table_cache | numeric | GLOBAL |
table_type | enumeration | GLOBAL | SESSION |
thread_cache_size | numeric | GLOBAL |
time_zone | string | GLOBAL | SESSION |
timestamp | numeric | SESSION |
tmp_table_size | numeric | GLOBAL | SESSION |
transaction_alloc_block_size | numeric | GLOBAL | SESSION |
transaction_prealloc_size | numeric | GLOBAL | SESSION |
tx_isolation | enumeration | GLOBAL | SESSION |
unique_checks | boolean | SESSION |
wait_timeout | numeric | GLOBAL | SESSION |
The server maintains many status variables that provide
information about its operation. You can view these variables and
their values by using the SHOW
STATUS statement (see Section 12.4.5.22, “SHOW STATUS Syntax”).
mysql> SHOW STATUS;
+--------------------------+------------+
| Variable_name | Value |
+--------------------------+------------+
| Aborted_clients | 0 |
| Aborted_connects | 0 |
| Bytes_received | 155372598 |
| Bytes_sent | 1176560426 |
| Connections | 30023 |
...
The following table lists all available server status variables:
Table 5.4 Status Variable Summary
Many status variables are reset to 0 by the FLUSH
STATUS statement.
The status variables have the following meanings. The
Com_ statement
counter variables were added beginning with MySQL 3.23.47. The
xxxQcache_ query
cache variables were added beginning with MySQL 4.0.1. Otherwise,
variables with no version indicated have been present since at
least MySQL 3.22.
xxx
For meanings of status variables specific to MySQL Cluster, see Section 15.3.4.4, “MySQL Cluster Status Variables”.
The number of connections that were aborted because the client died without closing the connection properly. See Section B.5.2.11, “Communication Errors and Aborted Connections”.
The number of failed attempts to connect to the MySQL server. See Section B.5.2.11, “Communication Errors and Aborted Connections”.
The number of transactions that used the temporary binary log
cache but that exceeded the value of
binlog_cache_size and used a
temporary file to store statements from the transaction. This
variable was added in MySQL 4.1.2.
The number of transactions that used the temporary binary log cache. This variable was added in MySQL 4.1.2.
The number of bytes received from all clients. This variable was added in MySQL 3.23.7.
The number of bytes sent to all clients. This variable was added in MySQL 3.23.7.
The Com_
statement counter variables were added beginning with MySQL
3.23.47. They indicate the number of times each
xxxxxx statement has been executed.
There is one status variable for each type of statement. For
example, Com_delete and
Com_insert count
DELETE and
INSERT statements,
respectively. However, if a query result is returned from
query cache, the server increments the
Qcache_hits status variable,
not Com_select. See
Section 7.5.3.4, “Query Cache Status and Maintenance”.
New Com_stmt_
status variables have been added in MySQL 4.1.13:
xxx
Com_stmt_prepare
Com_stmt_execute
Com_stmt_send_long_data
Com_stmt_reset
Com_stmt_close
Those variables stand for prepared statement commands. Their
names refer to the
COM_ command
set used in the network layer. In other words, their values
increase whenever prepared statement API calls such as
mysql_stmt_prepare(),
mysql_stmt_execute(), and so forth are
executed. However, xxxCom_stmt_prepare,
Com_stmt_execute and
Com_stmt_close also increase for
PREPARE,
EXECUTE, or
DEALLOCATE PREPARE,
respectively. Additionally, the values of the older (available
since MySQL 4.1.3) statement counter variables
Com_prepare_sql,
Com_execute_sql, and
Com_dealloc_sql increase for the
PREPARE,
EXECUTE, and
DEALLOCATE PREPARE statements.
All of the
Com_stmt_
variables are increased even if their argument (a prepared
statement) is unknown or an error occurred during execution;
in other words, their values correspond to the number of
requests issued, not to the number of requests successfully
completed.
xxx
The number of connection attempts (successful or not) to the MySQL server.
The number of internal on-disk temporary tables created by the server while executing statements. This variable was added in MySQL 3.23.24.
If an internal temporary table is created initially as an
in-memory table but becomes too large, MySQL automatically
converts it to an on-disk table. The maximum size for
in-memory temporary tables is the minimum of the
tmp_table_size and
max_heap_table_size values.
If Created_tmp_disk_tables
is large, you may want to increase the
tmp_table_size or
max_heap_table_size value to
lessen the likelihood that internal temporary tables in memory
will be converted to on-disk tables.
You can compare the number of internal on-disk temporary
tables created to the total number of internal temporary
tables created by comparing the values of the
Created_tmp_disk_tables and
Created_tmp_tables
variables.
See also Section 7.7.4, “How MySQL Uses Internal Temporary Tables”.
How many temporary files mysqld has created. This variable was added in MySQL 3.23.28.
The number of internal temporary tables created by the server while executing statements.
You can compare the number of internal on-disk temporary
tables created to the total number of internal temporary
tables created by comparing the values of the
Created_tmp_disk_tables and
Created_tmp_tables
variables.
See also Section 7.7.4, “How MySQL Uses Internal Temporary Tables”.
The number of rows written with INSERT
DELAYED for which some error occurred (probably
duplicate key).
The number of INSERT DELAYED
handler threads in use.
The number of INSERT DELAYED
rows written.
The number of executed FLUSH
statements.
The number of internal COMMIT
statements. This variable was added in MySQL 4.0.2.
The number of times a row was deleted from a table.
The number of times the first entry in an index was read. If
this value is high, it suggests that the server is doing a lot
of full index scans; for example, SELECT col1 FROM
foo, assuming that col1 is
indexed.
The number of requests to read a row based on a key. If this value is high, it is a good indication that your tables are properly indexed for your queries.
The number of requests to read the next row in key order. This value is incremented if you are querying an index column with a range constraint or if you are doing an index scan.
The number of requests to read the previous row in key order.
This read method is mainly used to optimize ORDER BY
... DESC. This variable was added in MySQL 3.23.6.
The number of requests to read a row based on a fixed position. This value is high if you are doing a lot of queries that require sorting of the result. You probably have a lot of queries that require MySQL to scan entire tables or you have joins that do not use keys properly.
The number of requests to read the next row in the data file. This value is high if you are doing a lot of table scans. Generally this suggests that your tables are not properly indexed or that your queries are not written to take advantage of the indexes you have.
The number of internal
ROLLBACK
statements. This variable was added in MySQL 4.0.2.
The number of requests to update a row in a table.
The number of requests to insert a row in a table.
The number of key blocks in the key cache that have changed
but have not yet been flushed to disk. This variable was added
in MySQL 4.1.1. It used to be known as
Not_flushed_key_blocks.
The number of unused blocks in the key cache. You can use this
value to determine how much of the key cache is in use; see
the discussion of
key_buffer_size in
Section 5.1.3, “Server System Variables”. This variable was
added in MySQL 4.1.2.
The number of used blocks in the key cache. This value is a high-water mark that indicates the maximum number of blocks that have ever been in use at one time.
The number of requests to read a key block from the cache.
The number of physical reads of a key block from disk. If
Key_reads is large, then
your key_buffer_size value is
probably too small. The cache miss rate can be calculated as
Key_reads/Key_read_requests.
The number of requests to write a key block to the cache.
The number of physical writes of a key block to disk.
The maximum number of connections that have been in use simultaneously since the server started.
The number of rows waiting to be written in
INSERT DELAYED queues.
The old name for
Key_blocks_not_flushed
before MySQL 4.1.1.
The number of files that are open. This count includes regular files opened by the server. It does not include other types of files such as sockets or pipes. Also, the count does not include files that storage engines open using their own internal functions rather than asking the server level to do so.
The number of streams that are open (used mainly for logging).
The number of tables that are open.
The number of tables that have been opened. If
Opened_tables is big, your
table_cache value is probably
too small.
The current number of prepared statements. (The maximum number
of statements is given by the
max_prepared_stmt_count
system variable.) This variable was added in MySQL 4.1.23.
The number of free memory blocks in the query cache.
The amount of free memory for the query cache.
The number of query cache hits.
The number of queries added to the query cache.
The number of queries that were deleted from the query cache because of low memory.
The number of noncached queries (not cacheable, or not cached
due to the query_cache_type
setting).
The number of queries registered in the query cache.
The total number of blocks in the query cache.
The number of statements that clients have sent to the server.
The status of fail-safe replication (not implemented).
The number of joins that perform table scans because they do not use indexes. If this value is not 0, you should carefully check the indexes of your tables. This variable was added in MySQL 3.23.25.
The number of joins that used a range search on a reference table. This variable was added in MySQL 3.23.25.
The number of joins that used ranges on the first table. This is normally not critical issue even if the value is quite large. This variable was added in MySQL 3.23.25.
The number of joins without keys that check for key usage
after each row. (If this is not equal to 0,
you should very carefully check the indexes of your tables.)
This variable was added in MySQL 3.23.25.
The number of joins that did a full scan of the first table. This variable was added in MySQL 3.23.25.
The number of temporary tables that the slave SQL thread currently has open. If the value is greater than zero, it is not safe to shut down the slave; see Section 14.7.12, “Replication and Temporary Tables”. This variable was added in MySQL 3.23.29.
Total (since startup) number of times the replication slave SQL thread has retried transactions. This variable was added in MySQL 4.1.11.
This is ON if this server is a replication
slave that is connected to a replication master, and both the
I/O and SQL threads are running; otherwise, it is
OFF.
This variable was added in MySQL 3.23.16.
The number of threads that have taken more than
slow_launch_time seconds to
create. This variable was added in MySQL 3.23.15.
The number of queries that have taken more than
long_query_time seconds. See
Section 5.3.5, “The Slow Query Log”.
The number of merge passes that the sort algorithm has had to
do. If this value is large, you should consider increasing the
value of the sort_buffer_size
system variable. This variable was added in MySQL 3.23.28.
The number of sorts that were done with ranges. This variable was added in MySQL 3.23.25.
The number of sorted rows. This variable was added in MySQL 3.23.25.
The number of sorts that were done by scanning the table. This variable was added in MySQL 3.23.25.
The number of negotiates needed to establish the connection. This variable was added in MySQL 4.0.0.
The number of accepted SSL connections. This variable was added in MySQL 4.0.0.
The number of callback cache hits. This variable was added in MySQL 4.0.0.
The current SSL cipher (empty for non-SSL connections). This variable was added in MySQL 4.0.0.
The list of possible SSL ciphers. This variable was added in MySQL 4.0.0.
The number of SSL connection attempts to an SSL-enabled master. This variable was added in MySQL 4.0.0.
The number of negotiates needed to establish the connection to an SSL-enabled master. This variable was added in MySQL 4.0.0.
The SSL context verification depth (how many certificates in the chain are tested). This variable was added in MySQL 4.0.0.
The SSL context verification mode. This variable was added in MySQL 4.0.0.
The default SSL timeout. This variable was added in MySQL 4.0.0.
The number of successful SSL connections to the server. This variable was added in MySQL 4.0.0.
The number of successful slave connections to an SSL-enabled master. This variable was added in MySQL 4.0.0.
The number of SSL session cache hits. This variable was added in MySQL 4.0.0.
The number of SSL session cache misses. This variable was added in MySQL 4.0.0.
The SSL session cache mode. This variable was added in MySQL 4.0.0.
The number of SSL session cache overflows. This variable was added in MySQL 4.0.0.
The SSL session cache size. This variable was added in MySQL 4.0.0.
The number of SSL session cache timeouts. This variable was added in MySQL 4.0.0.
How many SSL connections were reused from the cache. This variable was added in MySQL 4.0.0.
Ssl_used_session_cache_entries
How many SSL session cache entries were used. This variable was added in MySQL 4.0.0.
The verification depth for replication SSL connections. This variable was added in MySQL 4.0.0.
The verification mode for replication SSL connections. This variable was added in MySQL 4.0.0.
The SSL version number. This variable was added in MySQL 4.0.0.
The number of times that a request for a table lock could be granted immediately. This variable was added in MySQL 3.23.33.
The number of times that a request for a table lock could not be granted immediately and a wait was needed. If this is high and you have performance problems, you should first optimize your queries, and then either split your table or tables or use replication. This variable was added in MySQL 3.23.33.
The number of threads in the thread cache. This variable was added in MySQL 3.23.17.
The number of currently open connections.
The number of threads created to handle connections. If
Threads_created is big, you
may want to increase the
thread_cache_size value. The
cache miss rate can be calculated as
Threads_created divided by
Connections. This variable
was added in MySQL 3.23.31.
The number of threads that are not sleeping.
The number of seconds that the server has been up.
The MySQL server can operate in different SQL modes, and (as of MySQL 4.1) can apply these modes differentially for different clients. This capability enables each application to tailor the server's operating mode to its own requirements.
Modes define what SQL syntax MySQL should support and what kind of data validation checks it should perform. This makes it easier to use MySQL in different environments and to use MySQL together with other database servers.
You can set the default SQL mode by starting
mysqld with the
--sql-mode="
option, or by using
modes"sql-mode="
in modes"my.cnf (Unix operating systems) or
my.ini (Windows).
modes is a list of different modes
separated by comma (“,”)
characters. The default value is empty (no modes set). The
modes value also can be empty
(--sql-mode="" on the command line,
or sql-mode="" in
my.cnf on Unix systems or in
my.ini on Windows) if you want to clear it
explicitly.
Beginning with MySQL 4.1, you can change the SQL mode at runtime
by using a SET [GLOBAL|SESSION]
sql_mode=' statement to
set the modes'sql_mode system value.
Setting the GLOBAL variable requires the
SUPER privilege and affects the
operation of all clients that connect from that time on. Setting
the SESSION variable affects only the current
client. Any client can change its own session
sql_mode value at any time.
You can retrieve the current global or session
sql_mode value with the following
statements:
SELECT @@GLOBAL.sql_mode; SELECT @@SESSION.sql_mode;
This mode changes syntax and behavior to conform more closely to standard SQL, and is available beginning in MySQL 4.1.1.
The following list describes all supported modes:
Treat “"” as an identifier
quote character (like the “`”
quote character) and not as a string quote character. You can
still use “`” to quote
identifiers with this mode enabled. With
ANSI_QUOTES enabled, you
cannot use double quotation marks to quote literal strings,
because it is interpreted as an identifier. (Added in MySQL
4.0.0)
Permit spaces between a function name and the
“(” character. This causes
built-in function names to be treated as reserved words. As a
result, identifiers that are the same as function names must
be quoted as described in Section 8.2, “Database, Table, Index, Column, and Alias Names”. For
example, because there is a
COUNT() function, the use of
count as a table name in the following
statement causes an error:
mysql> CREATE TABLE count (i INT);
ERROR 1064 (42000): You have an error in your SQL syntax
The table name should be quoted:
mysql> CREATE TABLE `count` (i INT);
Query OK, 0 rows affected (0.00 sec)
The IGNORE_SPACE SQL mode
applies to built-in functions, not to user-defined functions.
It is always permissible to have spaces after a UDF name,
regardless of whether
IGNORE_SPACE is enabled.
For further discussion of
IGNORE_SPACE, see
Section 8.2.3, “Function Name Parsing and Resolution”.
(Added in MySQL 4.0.0)
NO_AUTO_VALUE_ON_ZERO
affects handling of AUTO_INCREMENT columns.
Normally, you generate the next sequence number for the column
by inserting either NULL or
0 into it.
NO_AUTO_VALUE_ON_ZERO
suppresses this behavior for 0 so that only
NULL generates the next sequence number.
(Added in MySQL 4.1.1)
This mode can be useful if 0 has been
stored in a table's AUTO_INCREMENT column.
(Storing 0 is not a recommended practice,
by the way.) For example, if you dump the table with
mysqldump and then reload it, MySQL
normally generates new sequence numbers when it encounters the
0 values, resulting in a table with
contents different from the one that was dumped. Enabling
NO_AUTO_VALUE_ON_ZERO before
reloading the dump file solves this problem. As of MySQL
4.1.1, mysqldump automatically includes a
statement in the dump output that enables
NO_AUTO_VALUE_ON_ZERO to
avoid this problem.
When creating a table, ignore all INDEX
DIRECTORY and DATA DIRECTORY
directives. This option is useful on slave replication
servers. (Added in MySQL 4.0.15)
Do not print MySQL-specific column options in the output of
SHOW CREATE TABLE. This mode is
used by mysqldump in portability mode.
(Added in MySQL 4.1.1)
Do not print MySQL-specific index options in the output of
SHOW CREATE TABLE. This mode is
used by mysqldump in portability mode.
(Added in MySQL 4.1.1)
Do not print MySQL-specific table options (such as
ENGINE) in the output of
SHOW CREATE TABLE. This mode is
used by mysqldump in portability mode.
(Added in MySQL 4.1.1)
By default, subtraction between integer operands produces an
UNSIGNED result if any operand
isUNSIGNED. When
NO_UNSIGNED_SUBTRACTION is
enabled, the subtraction result is signed, even if
any operand is unsigned. For example, compare the
type of column c2 in table
t1 with that of column
c2 in table t2:
mysql>SET sql_mode='';mysql>CREATE TABLE test (c1 BIGINT UNSIGNED NOT NULL);mysql>CREATE TABLE t1 SELECT c1 - 1 AS c2 FROM test;mysql>DESCRIBE t1;+-------+---------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------------------+------+-----+---------+-------+ | c2 | bigint(21) unsigned | | | 0 | | +-------+---------------------+------+-----+---------+-------+ mysql>SET sql_mode='NO_UNSIGNED_SUBTRACTION';mysql>CREATE TABLE t2 SELECT c1 - 1 AS c2 FROM test;mysql>DESCRIBE t2;+-------+------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+------------+------+-----+---------+-------+ | c2 | bigint(21) | | | 0 | | +-------+------------+------+-----+---------+-------+
Note that this means that BIGINT UNSIGNED
is not 100% usable in all contexts. See
Section 11.10, “Cast Functions and Operators”. (Added in MySQL 4.0.2)
mysql>SET sql_mode = '';mysql>SELECT CAST(0 AS UNSIGNED) - 1;+-------------------------+ | CAST(0 AS UNSIGNED) - 1 | +-------------------------+ | 18446744073709551615 | +-------------------------+ mysql>SET sql_mode = 'NO_UNSIGNED_SUBTRACTION';mysql>SELECT CAST(0 AS UNSIGNED) - 1;+-------------------------+ | CAST(0 AS UNSIGNED) - 1 | +-------------------------+ | -1 | +-------------------------+
Do not permit queries for which the
SELECT list refers to
nonaggregated columns that are not named in the GROUP
BY clause. (Added in MySQL 4.0.0) The following
query is invalid with this mode enabled because
address is not named in the GROUP
BY clause:
SELECT name, address, MAX(age) FROM t GROUP BY name;
Treat || as a
string concatenation operator (same as
CONCAT()) rather than as a
synonym for OR. (Added in MySQL
4.0.0)
Treat REAL as a synonym for
FLOAT. By default, MySQL treats
REAL as a synonym for
DOUBLE. (Added in MySQL 4.0.0)
The following special modes are provided as shorthand for combinations of mode values from the preceding list. All are available as of MySQL 4.1.1.
The descriptions include all mode values that are available in the most recent version of MySQL. For older versions, a combination mode does not include individual mode values that are not available except in newer versions.
Equivalent to REAL_AS_FLOAT,
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE. Before MySQL
4.1.11, ANSI also includes
ONLY_FULL_GROUP_BY. See
Section 1.9.3, “Running MySQL in ANSI Mode”.
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
Equivalent to
NO_FIELD_OPTIONS.
Equivalent to
NO_FIELD_OPTIONS.
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
Equivalent to
PIPES_AS_CONCAT,
ANSI_QUOTES,
IGNORE_SPACE,
NO_KEY_OPTIONS,
NO_TABLE_OPTIONS,
NO_FIELD_OPTIONS.
As of MySQL 4.1, MySQL Server supports a
HELP statement that returns online
information from the MySQL Reference manual (see
Section 12.7.3, “HELP Syntax”). The proper operation of this statement
requires that the help tables in the mysql
database be initialized with help topic information, which is done
by processing the contents of the
fill_help_tables.sql script.
If you install MySQL using a binary or source distribution on Unix, help table setup occurs when you run mysql_install_db. For an RPM distribution on Linux or binary distribution on Windows, help table setup occurs as part of the MySQL installation process.
If you upgrade MySQL using a binary distribution, the help tables
are not upgraded automatically, but you can upgrade them manually.
Locate the fill_help_tables.sql file in the
share or share/mysql
directory. Change location into that directory and process the
file with the mysql client as follows:
shell> mysql -u root mysql < fill_help_tables.sql
You can also obtain the latest
fill_help_tables.sql at any time to upgrade
your help tables. Download the proper file for your version of
MySQL from http://dev.mysql.com/doc/index-other.html. After
downloading and uncompressing the file, process it with
mysql as described previously.
If you are working with Bazaar and a MySQL development source
tree, you will need to download the
fill_help_tables.sql file because the tree
contains only a “stub” version.
On Unix, signals can be sent to processes. mysqld responds to signals sent to it as follows:
SIGTERM causes the server to shut down.
SIGHUP causes the server to reload the
grant tables and flush the logs (like
FLUSH
PRIVILEGES and
FLUSH LOGS).
It also writes a status report to the error log that has this
format:
Status information: Current dir: /var/mysql/data/ Running threads: 0 Stack size: 196608 Current locks: Key caches: default Buffer_size: 8388600 Block_size: 1024 Division_limit: 100 Age_limit: 300 blocks used: 0 not flushed: 0 w_requests: 0 writes: 0 r_requests: 0 reads: 0 handler status: read_key: 0 read_next: 0 read_rnd 0 read_first: 1 write: 0 delete 0 update: 0 Table status: Opened tables: 5 Open tables: 0 Open files: 7 Open streams: 0 Alarm status: Active alarms: 1 Max used alarms: 2 Next alarm time: 67
On some Mac OS X 10.3 versions, mysqld ignores
SIGHUP and SIGQUIT.
The server shutdown process takes place as follows:
The shutdown process is initiated.
Server shutdown can be initiated several ways. For example, a
user with the SHUTDOWN
privilege can execute a mysqladmin shutdown
command. mysqladmin can be used on any
platform supported by MySQL. Other operating system-specific
shutdown initiation methods are possible as well: The server
shuts down on Unix when it receives a
SIGTERM signal. A server running as a
service on Windows shuts down when the services manager tells
it to. (On Windows, a user with Administrator rights can also
shut down the server using NET STOP
, where
service_nameservice_name is the name of the
MySQL service. By default, this is MySQL.)
The server creates a shutdown thread if necessary.
Depending on how shutdown was initiated, the server might
create a thread to handle the shutdown process. If shutdown
was requested by a client, a shutdown thread is created. If
shutdown is the result of receiving a
SIGTERM signal, the signal thread might
handle shutdown itself, or it might create a separate thread
to do so. If the server tries to create a shutdown thread and
cannot (for example, if memory is exhausted), it issues a
diagnostic message that appears in the error log:
Error: Can't create thread to kill server
The server stops accepting new connections.
To prevent new activity from being initiated during shutdown, the server stops accepting new client connections. It does this by closing the network connections to which it normally listens for connections: the TCP/IP port, the Unix socket file, the Windows named pipe, and shared memory on Windows.
The server terminates current activity.
For each thread that is associated with a client connection,
the connection to the client is broken and the thread is
marked as killed. Threads die when they notice that they are
so marked. Threads for idle connections die quickly. Threads
that currently are processing statements check their state
periodically and take longer to die. For additional
information about thread termination, see
Section 12.4.6.3, “KILL Syntax”, in particular for the instructions
about killed REPAIR TABLE or
OPTIMIZE TABLE operations on
MyISAM tables.
For threads that have an open transaction, the transaction is
rolled back. Note that if a thread is updating a
nontransactional table, an operation such as a multiple-row
UPDATE or
INSERT may leave the table
partially updated, because the operation can terminate before
completion.
If the server is a master replication server, threads associated with currently connected slaves are treated like other client threads. That is, each one is marked as killed and exits when it next checks its state.
If the server is a slave replication server, the I/O and SQL threads, if active, are stopped before client threads are marked as killed. The SQL thread is permitted to finish its current statement (to avoid causing replication problems), and then stops. If the SQL thread was in the middle of a transaction at this point, the transaction is rolled back.
Storage engines are shut down or closed.
At this stage, the table cache is flushed and all open tables are closed.
Each storage engine performs any actions necessary for tables
that it manages. For example, MyISAM
flushes any pending index writes for a table.
InnoDB flushes its buffer pool to disk,
writes the current LSN to the tablespace, and terminates its
own internal threads.
The server exits.
A MySQL-Max server is a version of the mysqld MySQL server that has been built to include additional features. The MySQL-Max distribution to use depends on your platform:
For Windows, MySQL binary distributions include both the
standard server (mysqld.exe) and the
MySQL-Max server (mysqld-max.exe), so no
special distribution is needed. Just use a regular Windows
distribution. See Section 2.3, “Installing MySQL on Microsoft Windows”.
For Linux, if you install MySQL using RPM distributions, the
MySQL-Max RPM presupposes that you have
already installed the regular server RPM. Use the regular
MySQL-server RPM first to install a standard
server named mysqld, and then use the
MySQL-Max RPM to install a server named
mysqld-max. See
Section 2.4, “Installing MySQL from RPM Packages on Linux”, for more information
on the Linux RPM packages.
All other MySQL-Max distributions contain a single server that is named mysqld but that has the additional features included.
You can find the MySQL-Max binaries on the MySQL Web site at http://dev.mysql.com/doc/.
We build the MySQL-Max servers by using the following configure options:
--with-server-suffix=-max
This option adds a -max suffix to the
mysqld version string.
--with-innodb
This option enables support for the InnoDB
storage engine. MySQL-Max servers always include
InnoDB support, but this option actually is
needed only for MySQL 3.23. From MySQL 4.0 onward,
InnoDB is included by default in all binary
distributions, so a MySQL-Max server is not needed to obtain
InnoDB support.
--with-bdb
This option enables support for the Berkeley DB
(BDB) storage engine on those platforms for
which BDB is available. (See notes in the
following discussion.)
--with-blackhole-storage-engine
This option enables support for the BLACKHOLE
storage engine in MySQL 4.1.11 and newer.
--with-example-storage-engine
This option enables support for the EXAMPLE
storage engine in MySQL 4.1.10 and newer.
--with-ndbcluster
As of MySQL 4.1.2, this option enables support for the
NDBCLUSTER storage engine on those
platforms for which Cluster is available. (See notes in the
following discussion.)
USE_SYMDIR
This define is enabled to turn on database symbolic link support for Windows. This applies only before MySQL 4.0. From MySQL 4.0 onward, symbolic link support is enabled for all Windows servers, so a MySQL-Max server is not needed to take advantage of this feature.
MySQL-Max binary distributions are a convenience for those who wish to install precompiled programs. If you build MySQL using a source distribution, you can build your own Max-like server by enabling the same features at configuration time that the MySQL-Max binary distributions are built with.
MySQL-Max servers include the BerkeleyDB (BDB)
storage engine whenever possible, but not all platforms support
BDB.
The following table shows on which platforms MySQL-Max binaries
include support for BDB and NDB
Cluster:
As of MySQL 4.1.2, MySQL Cluster is supported on Linux (on most
platforms), Solaris, Mac OS X, and HP-UX only. Some users have
reported success in using MySQL Cluster built from source on BSD
operating systems, but these are not officially supported at this
time. Note that, even for servers compiled with Cluster support, the
NDBCLUSTER storage engine is not
enabled by default. You must start the server with the
--ndbcluster option to use it as part
of a MySQL Cluster. (For details, see
Section 15.3, “MySQL Cluster Configuration”.)
The following table shows the platforms for which MySQL-Max binaries
include support for BDB and
NDBCLUSTER.
| System | BDB Support | NDB Support |
|---|---|---|
| AIX 5.2 | N | N |
| HP-UX | Y | Y |
| Linux-Alpha | N | N |
| Linux-IA-64 | N | Y |
| Linux-Intel | Y | Y |
| Mac OS X | N | Y |
| NetWare | N | N |
| SCO 6 | N | N |
| Solaris-SPARC | Y | Y |
| Solaris-Intel | N | Y |
| Solaris-AMD 64 | Y | Y |
| Windows NT/2000/XP | Y | N |
To find out which storage engines your server supports, use the
SHOW ENGINES statement. (See
Section 12.4.5.10, “SHOW ENGINES Syntax”.) For example:
mysql> SHOW ENGINES\G
*************************** 1. row ***************************
Engine: MyISAM
Support: DEFAULT
Comment: Default engine as of MySQL 3.23 with great performance
*************************** 2. row ***************************
Engine: HEAP
Support: YES
Comment: Alias for MEMORY
*************************** 3. row ***************************
Engine: MEMORY
Support: YES
Comment: Hash based, stored in memory, useful for temporary tables
*************************** 4. row ***************************
Engine: MERGE
Support: YES
Comment: Collection of identical MyISAM tables
...
Before MySQL 4.1.2, SHOW ENGINES is
unavailable. Use the following statement instead and check the value
of the variable for the storage engine in which you are interested:
mysql> SHOW VARIABLES LIKE 'have%';
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| have_archive | YES |
| have_bdb | YES |
| have_blackhole_engine | YES |
| have_compress | YES |
| have_crypt | YES |
| have_csv | YES |
| have_example_engine | YES |
| have_geometry | YES |
| have_innodb | YES |
| have_isam | NO |
| have_ndbcluster | NO |
| have_openssl | YES |
| have_query_cache | YES |
| have_raid | NO |
| have_rtree_keys | YES |
| have_symlink | YES |
+-----------------------+-------+
16 rows in set (0.00 sec)
The precise output from these statements may vary according to the MySQL version used (and the features that are enabled). The values of the second column of the output indicate the server's level of support for each feature, as shown here:
| Value | Meaning |
|---|---|
YES | The feature is supported and is active. |
NO | The feature is not supported. |
DISABLED | The feature is supported but has been disabled. |
A value of NO means that the server was compiled
without support for the feature, so it cannot be activated at
runtime.
A value of DISABLED occurs either because the
server was started with an option that disables the feature, or
because not all options required to enable it were given. In the
latter case, the error log file should contain a reason indicating
why the option is disabled. See Section 5.3.1, “The Error Log”.
One situation in which you might see DISABLED
occurs with MySQL 3.23 when the InnoDB storage
engine is compiled in. In MySQL 3.23, you must supply at least the
innodb_data_file_path option at
runtime to set up the InnoDB tablespace. Without
this option, InnoDB disables itself. See
Section 13.2.2, “InnoDB in MySQL 3.23”. You can specify
configuration options for the BDB storage engine,
too, but BDB does not disable itself if you do
not provide them. See Section 13.5.3, “BDB Startup Options”.
You might also see DISABLED for a storage engine
if the server was compiled to support it, but was started with a
--skip-
option. For example,
engine_name--skip-innodb
disables the InnoDB engine. For the NDB
Cluster storage engine, DISABLED means
the server was compiled with support for MySQL Cluster, but was not
started with the --ndb-cluster option.
As of version 3.23, all MySQL servers support
MyISAM tables, because MyISAM
is the default storage engine.
MySQL Server has several logs that can help you find out what activity is taking place.
| Log Type | Information Written to Log |
|---|---|
| Error log | Problems encountered starting, running, or stopping mysqld |
| ISAM log | Changes to the ISAM tables (used only for debugging
the ISAM code) |
| General query log | Established client connections and statements received from clients |
| Update log | Statements that change data (this log is deprecated) |
| Binary log | Statements that change data (also used for replication) |
| Relay log | Data changes received from a replication master server |
| Slow query log | Queries that took more than
long_query_time seconds to
execute |
By default, all log files are created in the data directory. You can
force the server to close and reopen the log files (or in some cases
switch to a new log file) by flushing the logs. Log flushing occurs
when you issue a FLUSH
LOGS statement or execute a mysqladmin
flush-logs, mysqladmin refresh,
mysqldump --flush-logs, or mysqldump
--master-data command. See Section 12.4.6.2, “FLUSH Syntax”,
Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”, and Section 4.5.4, “mysqldump — A Database Backup Program”. In
addition, the binary log is flushed when its size reaches the value
of the max_binlog_size system
variable.
The relay log is used only on slave replication servers, to hold data changes from the master server that must also be made on the slave. For discussion of relay log contents and configuration, see Section 14.3.2, “The Slave Relay Log”.
For information about log maintenance operations such as expiration of old log files, see Section 5.3.6, “Server Log Maintenance”.
See Section 5.4.2.1, “Administrator Guidelines for Password Security”, for information about keeping logs secure.
The error log contains information indicating when mysqld was started and stopped and also any critical errors that occur while the server is running. If mysqld notices a table that needs to be automatically checked or repaired, it writes a message to the error log.
On some operating systems, the error log contains a stack trace if mysqld dies. The trace can be used to determine where mysqld died. See Section 18.4, “Porting to Other Systems”.
Beginning with MySQL 4.0.10, you can specify where
mysqld writes the error log with the
--log-error[=
option. If the option is given with no
file_name]file_name value,
mysqld uses the name
host_name.errmysql.err.) If you flush the logs using
FLUSH LOGS or
mysqladmin flush-logs,
mysqld renames the current log file with the
suffix -old, then creates a new empty log file.
Be aware that a second log-flushing operation thus causes the
original error log file to be lost unless you save it under a
different name. For example, you can use the following commands to
save the file:
shell>mysqladmin flush-logsshell>mvhost_name.err-oldbackup-directory
No error log renaming occurs when the logs are flushed if the server is not writing to a named file.
In older MySQL versions on Unix, error log handling was done by
mysqld_safe which redirected the error file to
host_name.err--err-log=
option to mysqld_safe.
file_name
If you do not specify --log-error,
or (on Windows) if you use the
--console option, errors are
written to stderr, the standard error output.
Usually this is your terminal.
On Windows, error output is always written to the
.err file if
--console is not given.
In addition, on Windows, events and error messages are written to
the Windows Event Log within the Application log. Entries marked
as Warning and Note are
written to the Event Log, but informational messages (such as
information statements from individual storage engines) are not
copied to the Event Log. The log entries have a source of
MySQL. You cannot disable writing information
to the Windows Event Log.
The --log-warnings option or
log_warnings system variable can
be used to control warning logging to the error log. The default
value is enabled (1) as of MySQL 4.0.19 and 4.1.2. Warning logging
can be disabled using a value of 0. As of MySQL 4.0.21 and 4.1.3,
the value can be greater than 1. If the value is greater than 1,
aborted connections are written to the error log. See
Section B.5.2.11, “Communication Errors and Aborted Connections”.
If mysqld_safe is used to start
mysqld and mysqld dies
unexpectedly, mysqld_safe notices that it needs
to restart mysqld and writes a
restarted mysqld message to the error log.
The general query log is a general record of what mysqld is doing. The server writes information to this log when clients connect or disconnect, and it logs each SQL statement received from clients. The general query log can be very useful when you suspect an error in a client and want to know exactly what the client sent to mysqld.
Older versions of the mysql.server script (from
MySQL 3.23.4 to 3.23.8) pass a
--log option to
safe_mysqld to enable the general query log. If
you need better performance when you start using MySQL in a
production environment, you can remove the
--log option from
mysql.server or change it to
--log-bin. See
Section 5.3.4, “The Binary Log”.
mysqld writes statements to the query log in the order that it receives them, which might differ from the order in which they are executed. This logging order contrasts to the update log and the binary log, which are written after the query is executed but before any locks are released. (Also, the query log contains all statements, whereas the update and binary logs do not contain statements that only select data.)
To enable the general query log, start mysqld
with the
--log[=
or file_name]-l [
option.
file_name]
If the general query log file is enabled but no name is specified,
the default name is
host_name.log
Server restarts and log flushing do not cause a new general query log file to be generated (although flushing closes and reopens it). On Unix, you can rename the file and create a new one by using the following commands:
shell>mvshell>host_name.loghost_name-old.logmysqladmin flush-logsshell>mvhost_name-old.logbackup-directory
On Windows, you cannot rename the log file while the server has it open. You must stop the server, rename the file, and then restart the server to create a new log file.
To disable or enable general query logging for the current
connection, set the session
sql_log_off variable to
ON or OFF.
The general query log should be protected because logged statements might contain passwords. See Section 5.4.2.1, “Administrator Guidelines for Password Security”.
The update log has been deprecated and replaced by the more useful, informative, and efficient binary log. See Section 5.3.4, “The Binary Log”.
When started with the
--log-update[=
option, mysqld writes a log file containing all
SQL statements that update data. If no
file_name]file_name value is given, the default
name is name of the host machine. If a file name is given, but it
does not contain a leading path, the file is written in the data
directory. If file_name does not have an
extension, mysqld creates log files with names
of the form file_name.nnnnnn, where
nnnnnn is a number that is incremented
each time you start the server or flush the logs.
For this naming scheme to work, you must not create your own files with the same names as those that might be used in the log file sequence.
Update logging is “smart” in that it logs
only statements that actually update data. Thus, an
UPDATE or
DELETE with a
WHERE clause that finds no rows is not written
to the log. Update logging also skips
UPDATE statements that merely set a
column to its existing value.
The update logging is done immediately after a query completes but before any locks are released or any commit is done. This ensures that statements are logged in execution order.
If you want to update a database from update log files, you could
do the following (assuming that your update logs have names of the
form file_name.nnnnnn):
shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql
ls is used to sort the update log file names into the right order.
This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.
The update log should be protected because logged statements might contain passwords. See Section 5.4.2.1, “Administrator Guidelines for Password Security”.
The binary log contains “events” that describe
database changes such as table creation operations or changes to
table data. As of MySQL 4.1.3, it also contains events for
statements that potentially could have made changes (for example,
a DELETE which matched no rows).
The binary log also contains information about how long each
statement took that updated data. The binary log has two important
purposes:
For replication, the binary log is used on master replication servers as a record of the statements to be sent to slave servers. The master server sends the events contained in its binary log to its slaves, which execute those events to make the same data changes that were made on the master. See Section 14.2, “Replication Implementation Overview”.
Certain data recovery operations require use of the binary log. After a backup has been restored, the events in the binary log that were recorded after the backup was made are re-executed. These events bring databases up to date from the point of the backup. See Section 6.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.
The binary log has replaced the old update log, which is no longer available as of MySQL 5.0. The binary log contains all information that is available in the update log in a more efficient format and in a manner that is transaction-safe. If you are using transactions, you must use the MySQL binary log for backups instead of the old update log.
Running a server with binary logging enabled makes performance slightly slower. However, the benefits of the binary log in enabling you to set up replication and for restore operations generally outweigh this minor performance decrement.
For information about server options and variables affecting the operation of binary logging, see Section 14.8.4, “Binary Log Options and Variables”.
The binary log is not used for statements such as
SELECT or
SHOW that do not modify data. If
you want to log all statements (for example, to identify a problem
query), use the general query log. See
Section 5.3.2, “The General Query Log”.
The binary log should be protected because logged statements might contain passwords. See Section 5.4.2.1, “Administrator Guidelines for Password Security”.
For detailed information about the format of the binary log, see MySQL Internals: The Binary Log.
To enable the binary log, start the server with the
--log-bin[=
option. If no base_name]base_name value is given,
the default name is the value of the pid-file
option (which by default is the name of host machine) followed by
-bin. If the basename is given, the server
writes the file in the data directory unless the basename is given
with a leading absolute path name to specify a different
directory. It is recommended that you specify a basename; see
Section B.5.8.4, “Open Issues in MySQL”, for the reason.
If you supply an extension in the log name (for example,
--log-bin=),
the extension is silently removed and ignored.
base_name.extension
mysqld appends a numeric extension to the
binary log basename to generate binary log file names. The number
increases each time the server creates a new log file, thus
creating an ordered series of files. The server creates a new file
in the series each time it starts or flushes the logs. The server
also creates a new binary log file automatically after the current
log's size reaches
max_binlog_size. A binary log
file may become larger than
max_binlog_size if you are using
large transactions because a transaction is written to the file in
one piece, never split between files.
To keep track of which binary log files have been used,
mysqld also creates a binary log index file
that contains the names of all used binary log files. By default,
this has the same basename as the binary log file, with the
extension '.index'. You can change the name of
the binary log index file with the
--log-bin-index[=
option. You should not manually edit this file while
mysqld is running; doing so would confuse
mysqld.
file_name]
The term “binary log file” generally denotes an individual numbered file containing database events. The term “binary log” collectively denotes the set of numbered binary log files plus the index file.
The server evaluates the
--binlog-do-db and
--binlog-ignore-db options in the
same way as it does the
--replicate-do-db and
--replicate-ignore-db options. For
information about how this is done, see
Section 14.9.1, “Evaluation of Database-Level Replication and Binary Logging Options”.
A replication slave server by default does not write to its own
binary log any data modifications that are received from the
replication master. To log these modifications, start the slave
with the --log-slave-updates option
in addition to the --log-bin option
(see Section 14.8.3, “Replication Slave Options and Variables”). This is done
when a slave is also to act as a master to other slaves in chained
replication.
You can delete all binary log files with the
RESET MASTER statement, or a subset
of them with PURGE BINARY LOGS. See
Section 12.4.6.5, “RESET Syntax”, and Section 12.5.1.1, “PURGE BINARY LOGS Syntax”.
If you are using replication, you should not delete old binary log
files on the master until you are sure that no slave still needs
to use them. For example, if your slaves never run more than three
days behind, once a day you can execute mysqladmin
flush-logs on the master and then remove any logs that
are more than three days old. You can remove the files manually,
but it is preferable to use PURGE BINARY
LOGS, which also safely updates the binary log index
file for you (and which can take a date argument as of MySQL 4.1).
See Section 12.5.1.1, “PURGE BINARY LOGS Syntax”.
A client that has the SUPER
privilege can disable binary logging of its own statements by
using a SET sql_log_bin=0 statement. See
Section 5.1.3, “Server System Variables”.
You can display the contents of binary log files with the mysqlbinlog utility. This can be useful when you want to reprocess statements in the log for a recovery operation. For example, you can update a MySQL server from the binary log as follows:
shell> mysqlbinlog log_file | mysql -h server_name
mysqlbinlog also can be used to display replication slave relay log file contents because they are written using the same format as binary log files. For more information on the mysqlbinlog utility and how to use it, see Section 4.6.6, “mysqlbinlog — Utility for Processing Binary Log Files”. For more information about the binary log and recovery operations, see Section 6.5, “Point-in-Time (Incremental) Recovery Using the Binary Log”.
Binary logging is done immediately after a statement completes but before any locks are released or any commit is done. This ensures that the log is logged in execution order.
Updates to nontransactional tables are stored in the binary log
immediately after execution. Within an uncommitted transaction,
all updates (UPDATE,
DELETE, or
INSERT) that change transactional
tables such as BDB or InnoDB
tables are cached until a COMMIT
statement is received by the server. At that point,
mysqld writes the entire transaction to the
binary log before the COMMIT is
executed.
Modifications to nontransactional tables cannot be rolled back. If
a transaction that is rolled back includes modifications to
nontransactional tables, the entire transaction is logged with a
ROLLBACK
statement at the end to ensure that the modifications to those
tables are replicated. This is true as of MySQL 4.0.15.
When a thread that handles the transaction starts, it allocates a
buffer of binlog_cache_size to
buffer statements. If a statement is bigger than this, the thread
opens a temporary file to store the transaction. The temporary
file is deleted when the thread ends.
The Binlog_cache_use status
variable shows the number of transactions that used this buffer
(and possibly a temporary file) for storing statements. The
Binlog_cache_disk_use status
variable shows how many of those transactions actually had to use
a temporary file. These two variables can be used for tuning
binlog_cache_size to a large
enough value that avoids the use of temporary files.
The max_binlog_cache_size system
variable (default 4GB, which is also the maximum) can be used to
restrict the total size used to cache a multiple-statement
transaction. If a transaction is larger than this many bytes, it
fails and rolls back. The minimum value is 4096.
If you are using the update log or binary log, concurrent inserts
are converted to normal inserts for CREATE ...
SELECT or
INSERT ...
SELECT statements. This is done to ensure that you can
re-create an exact copy of your tables by applying the log during
a backup operation.
The binary log format has some known limitations that can affect
recovery from backups, especially in old versions. These caveats,
which also affect replication, are listed at
Section 14.7, “Replication Features and Issues”. One caveat which does not
affect replication but only recovery with
mysqlbinlog: before MySQL 4.1,
mysqlbinlog could not prepare output suitable
for mysql if the binary log contained
interlaced statements originating from different clients that used
temporary tables of the same name. This is fixed in MySQL 4.1.
However, the problem still existed for
LOAD DATA
INFILE statements until it was fixed in MySQL 4.1.8.
The binary log format differs between versions 3.23 and 4.0. (These format changes were required to implement enhancements to replication.) However, MySQL 4.1 has the same binary log format as 4.0. See Section 14.5, “Replication Compatibility Between MySQL Versions”.
Before MySQL 4.1.9, writes to a binary log file or binary log
index file that failed due to a full disk or an exceeded quota
resulted in corruption of the file. Starting from MySQL 4.1.9,
writes to the binary log file and binary log index file are
handled the same way as writes to MyISAM
tables. See Section B.5.4.3, “How MySQL Handles a Full Disk”.
By default, the binary log is not synchronized to disk at each
write. So if the operating system or machine (not only the MySQL
server) crashes, there is a chance that the last statements of the
binary log are lost. To prevent this, you can make the binary log
be synchronized to disk after every N
writes to the binary log, with the
sync_binlog system variable. See
Section 5.1.3, “Server System Variables”. 1 is the safest value
for sync_binlog, but also the
slowest. Even with sync_binlog
set to 1, there is still the chance of an inconsistency between
the table content and binary log content in case of a crash. For
example, if you are using InnoDB tables and the
MySQL server processes a COMMIT
statement, it writes the whole transaction to the binary log and
then commits this transaction into InnoDB. If
the server crashes between those two operations, the transaction
is rolled back by InnoDB at restart but still
exists in the binary log. This problem can be solved with the
--innodb-safe-binlog option
(available starting from MySQL 4.1.3), which adds consistency
between the content of InnoDB tables and the
binary log.
For this option to provide a greater degree of safety, the MySQL
server should also be configured to synchronize the binary log and
the InnoDB logs to disk at every transaction.
The InnoDB logs are synchronized by default,
and sync_binlog=1 can be used to synchronize
the binary log. The effect of this option is that at restart after
a crash, after doing a rollback of transactions, the MySQL server
cuts rolled back InnoDB transactions from the
binary log. This ensures that the binary log reflects the exact
data of InnoDB tables, and so, that the slave
remains in synchrony with the master (not receiving a statement
which has been rolled back).
Note that --innodb-safe-binlog can
be used even if the MySQL server updates other storage engines
than InnoDB. Only statements and transactions
that affect InnoDB tables are subject to
removal from the binary log at InnoDB's crash
recovery. If the MySQL server discovers at crash recovery that the
binary log is shorter than it should have been, it lacks at least
one successfully committed InnoDB transaction.
This should not happen if sync_binlog=1 and the
disk/file system do an actual sync when they are requested to
(some do not), so the server prints an error message The
binary log . In this case, this binary log is not
correct and replication should be restarted from a fresh snapshot
of the master's data.
file_name is shorter than its
expected size
The slow query log consists of SQL statements that took more than
long_query_time seconds to
execute. The minimum and default values of
long_query_time are 1 and 10,
respectively.
The time to acquire the initial table locks is not counted as execution time. mysqld writes a statement to the slow query log after it has been executed and after all locks have been released, so log order might differ from execution order.
To enable the slow query log, start mysqld with
the
--log-slow-queries[=
option.
file_name]
If the slow query log file is enabled but no name is specified,
the default name is
host_name-slow.log
Before MySQL 4.1, if you also use
--log-long-format when logging slow
queries, queries that are not using indexes are logged as well.
Starting with MySQL 4.1, logging of queries not using indexes for
row lookups is enabled using the
--log-queries-not-using-indexes
option instead. The
--log-long-format is deprecated as
of MySQL 4.1, when
--log-short-format was introduced,
which causes less information to be logged. (The long log format
is the default setting since version 4.1.) See
Section 5.1.2, “Server Command Options”.
In MySQL 4.0, slow administrative statements such as
OPTIMIZE TABLE,
ANALYZE TABLE, and
ALTER TABLE were written to the
slow query log. This logging was disabled in MySQL 4.1 until
4.1.13, when the
--log-slow-admin-statements server
option was added to specify logging of slow administrative
statements.
The server does not write queries handled by the query cache to the slow query log, nor queries that would not benefit from the presence of an index because the table has zero rows or one row.
Replication slaves do not write replicated queries to the slow query log, even if the same queries were written to the slow query log on the master. This is a known issue. (Bug #23300)
The slow query log should be protected because logged statements might contain passwords. See Section 5.4.2.1, “Administrator Guidelines for Password Security”.
The slow query log can be used to find queries that take a long time to execute and are therefore candidates for optimization. However, examining a long slow query log can become a difficult task. To make this easier, you can process a slow query log file using the mysqldumpslow command to summarize the queries that appear in the log. See Section 4.6.7, “mysqldumpslow — Summarize Slow Query Log Files”.
As described in Section 5.3, “MySQL Server Logs”, MySQL Server can create several different log files to help you see what activity is taking place. However, you must clean up these files regularly to ensure that the logs do not take up too much disk space.
When using MySQL with logging enabled, you may want to back up and remove old log files from time to time and tell MySQL to start logging to new files. See Section 6.2, “Database Backup Methods”.
On a Linux (Red Hat) installation, you can use the
mysql-log-rotate script for this. If you
installed MySQL from an RPM distribution, this script should have
been installed automatically. Be careful with this script if you
are using the binary log for replication. You should not remove
binary logs until you are certain that their contents have been
processed by all slaves.
On other systems, you must install a short script yourself that you start from cron (or its equivalent) for handling log files.
For the binary log, you can set the
expire_logs_days system variable
to expire binary log files automatically after a given number of
days (see Section 5.1.3, “Server System Variables”). If you are
using replication, you should set the variable no lower than the
maximum number of days your slaves might lag behind the master. To
remove binary logs on demand, use the PURGE
BINARY LOGS statement (see
Section 12.5.1.1, “PURGE BINARY LOGS Syntax”).
You can force MySQL to start using new log files by flushing the
logs. Log flushing occurs when you issue a
FLUSH LOGS
statement or execute a mysqladmin flush-logs,
mysqladmin refresh, mysqldump
--flush-logs, or mysqldump
--master-data command. See Section 12.4.6.2, “FLUSH Syntax”,
Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”, and Section 4.5.4, “mysqldump — A Database Backup Program”. In
addition, the binary log is flushed when its size reaches the
value of the max_binlog_size
system variable.
A log-flushing operation does the following:
If general query logging
(--log) or slow query logging
(--log-slow-queries) to a log
file is enabled, the server closes and reopens the general
query log file or slow query log file.
If update logging
(--log-update) or binary
logging (--log-bin) is used,
the server closes the log and opens a new log file with a
higher sequence number.
If the server was started with the
--log-error option to cause the
error log to be written to a file, it renames the current log
file with the suffix -old and creates a new
empty error log file.
The server creates a new binary log file when you flush the logs.
However, it just closes and reopens the general and slow query log
files. To cause new files to be created on Unix, rename the
current log files before flushing them. At flush time, the server
opens new log files with the original names. For example, if the
general and slow query log files are named
mysql.log and
mysql-slow.log, you can use a series of
commands like this:
shell>cdshell>mysql-data-directorymv mysql.log mysql.oldshell>mv mysql-slow.log mysql-slow.oldshell>mysqladmin flush-logs
On Windows, use rename rather than mv.
At this point, you can make a backup of
mysql.old and
mysql-slow.old and then remove them from
disk.
On Windows, you cannot rename log files while the server has them open. You must stop the server and rename them, and then restart the server to create new logs. For the error log, you can rename the file without a restart as described in Section 5.3.1, “The Error Log”.
This section describes some general security issues to be aware of and what you can do to make your MySQL installation more secure against attack or misuse. For information specifically about the access control system that MySQL uses for setting up user accounts and checking database access, see Section 5.5, “The MySQL Access Privilege System”.
Anyone using MySQL on a computer connected to the Internet should read this section to avoid the most common security mistakes.
In discussing security, we emphasize the necessity of fully protecting the entire server host (not just the MySQL server) against all types of applicable attacks: eavesdropping, altering, playback, and denial of service. We do not cover all aspects of availability and fault tolerance here.
MySQL uses security based on Access Control Lists (ACLs) for all connections, queries, and other operations that users can attempt to perform. There is also support for SSL-encrypted connections between MySQL clients and servers. Many of the concepts discussed here are not specific to MySQL at all; the same general ideas apply to almost all applications.
When running MySQL, follow these guidelines whenever possible:
Do not ever give anyone (except MySQL
root accounts) access to the
user table in the mysql
database! This is critical, particularly before
MySQL 4.1, when the encrypted password is the real
password in MySQL: Anyone who knows the password
that is listed in the mysql.user table and
who has access to the host listed for the account
can easily log in as that user. In MySQL
4.1, the password hashing algorithm was changed so that this
is no longer true.
Learn the MySQL access privilege system. The
GRANT and
REVOKE statements are used for
controlling access to MySQL. Do not grant more privileges than
necessary. Never grant privileges to all hosts.
Checklist:
Try mysql -u root. If you are able to
connect successfully to the server without being asked for
a password, anyone can connect to your MySQL server as the
MySQL root user with full privileges!
Review the MySQL installation instructions, paying
particular attention to the information about setting a
root password. See
Section 2.10.3, “Securing the Initial MySQL Accounts”.
Use the SHOW GRANTS
statement to check which accounts have access to what.
Then use the REVOKE
statement to remove those privileges that are not
necessary.
Do not store any plaintext passwords in your database. If your
computer becomes compromised, the intruder can take the full
list of passwords and use them. Instead, use
MD5(),
SHA1(), or some other one-way
hashing function and store the hash value.
Do not choose passwords from dictionaries. Special programs exist to break passwords. Even passwords like “xfish98” are very bad. Much better is “duag98” which contains the same word “fish” but typed one key to the left on a standard QWERTY keyboard. Another method is to use a password that is taken from the first characters of each word in a sentence (for example, “Mary had a little lamb” results in a password of “Mhall”). The password is easy to remember and type, but difficult to guess for someone who does not know the sentence.
Invest in a firewall. This protects you from at least 50% of all types of exploits in any software. Put MySQL behind the firewall or in a demilitarized zone (DMZ).
Checklist:
Try to scan your ports from the Internet using a tool such
as nmap. MySQL uses port 3306 by
default. This port should not be accessible from untrusted
hosts. Another simple way to check whether or not your
MySQL port is open is to try the following command from
some remote machine, where
server_host is the host name or
IP address of the host on which your MySQL server runs:
shell> telnet server_host 3306
If you get a connection and some garbage characters, the port is open, and should be closed on your firewall or router, unless you really have a good reason to keep it open. If telnet hangs or the connection is refused, the port is blocked, which is how you want it to be.
Do not trust any data entered by users of your applications.
They can try to trick your code by entering special or escaped
character sequences in Web forms, URLs, or whatever
application you have built. Be sure that your application
remains secure if a user enters something like
“; DROP DATABASE mysql;”. This
is an extreme example, but large security leaks and data loss
might occur as a result of hackers using similar techniques,
if you do not prepare for them.
A common mistake is to protect only string data values.
Remember to check numeric data as well. If an application
generates a query such as SELECT * FROM table WHERE
ID=234 when a user enters the value
234, the user can enter the value
234 OR 1=1 to cause the application to
generate the query SELECT * FROM table WHERE ID=234
OR 1=1. As a result, the server retrieves every row
in the table. This exposes every row and causes excessive
server load. The simplest way to protect from this type of
attack is to use single quotation marks around the numeric
constants: SELECT * FROM table WHERE
ID='234'. If the user enters extra information, it
all becomes part of the string. In a numeric context, MySQL
automatically converts this string to a number and strips any
trailing nonnumeric characters from it.
Sometimes people think that if a database contains only publicly available data, it need not be protected. This is incorrect. Even if it is permissible to display any row in the database, you should still protect against denial of service attacks (for example, those that are based on the technique in the preceding paragraph that causes the server to waste resources). Otherwise, your server becomes unresponsive to legitimate users.
Checklist:
Try to enter single and double quotation marks
(“'” and
“"”) in all of your Web
forms. If you get any kind of MySQL error, investigate the
problem right away.
Try to modify dynamic URLs by adding
%22
(“"”),
%23
(“#”), and
%27
(“'”) to them.
Try to modify data types in dynamic URLs from numeric to character types using the characters shown in the previous examples. Your application should be safe against these and similar attacks.
Try to enter characters, spaces, and special symbols rather than numbers in numeric fields. Your application should remove them before passing them to MySQL or else generate an error. Passing unchecked values to MySQL is very dangerous!
Check the size of data before passing it to MySQL.
Have your application connect to the database using a user name different from the one you use for administrative purposes. Do not give your applications any access privileges they do not need.
Many application programming interfaces provide a means of escaping special characters in data values. Properly used, this prevents application users from entering values that cause the application to generate statements that have a different effect than you intend:
MySQL C API: Use the
mysql_real_escape_string()
API call.
MySQL++: Use the escape and
quote modifiers for query streams.
PHP: Use the
mysql_real_escape_string() function
(available as of PHP 4.3.0, prior to that PHP version use
mysql_escape_string(), and prior to
PHP 4.0.3, use addslashes() ). Note
that only mysql_real_escape_string()
is character set-aware; the other functions can be
“bypassed” when using (invalid) multi-byte
character sets. In PHP 5 (and as of MySQL 4.1), you can
use the mysqli extension, which
supports the improved MySQL authentication protocol and
passwords, as well as prepared statements with
placeholders.
Perl DBI: Use placeholders or the
quote() method.
Ruby DBI: Use placeholders or the
quote() method.
Java JDBC: Use a PreparedStatement
object and placeholders.
Other programming interfaces might have similar capabilities.
Do not transmit plain (unencrypted) data over the Internet. This information is accessible to everyone who has the time and ability to intercept it and use it for their own purposes. Instead, use an encrypted protocol such as SSL or SSH. MySQL supports internal SSL connections as of version 4.0. Another technique is to use SSH port-forwarding to create an encrypted (and compressed) tunnel for the communication.
Learn to use the tcpdump and strings utilities. In most cases, you can check whether MySQL data streams are unencrypted by issuing a command like the following:
shell> tcpdump -l -i eth0 -w - src or dst port 3306 | strings
This works under Linux and should work with small modifications under other systems.
If you do not see plaintext data, this does not always mean that the information actually is encrypted. If you need high security, you should consult with a security expert.
Passwords occur in several contexts within MySQL. The following sections provide guidelines that enable administrators and end users to keep these passwords secure and avoid exposing them. There is also a discussion of how MySQL uses password hashing internally.
Database administrators should use the following guidelines to keep passwords secure.
MySQL stores passwords for user accounts in the
mysql.user table. Access to this table should
never be granted to any nonadministrative accounts. Passwords in
the user table are stored in encrypted form,
but in versions of MySQL earlier than 4.1, knowing the encrypted
password for an account makes it possible to connect to the
server using that account.
Passwords can appear as plain text in SQL statements such as
GRANT and
SET PASSWORD, or statements that
invoke the PASSWORD() function.
If these statements are logged by the MySQL server, the
passwords become available to anyone with access to the logs.
This applies to the general query log, the slow query log, the
update log, and the binary log (see
Section 5.3, “MySQL Server Logs”). To guard against unwarranted
exposure to log files, they should be located in a directory
that restricts access to only the server and the database
administrator.
Replication slaves store the password for the replication master
in the master.info file. Access to this
file should be restricted to the database adminstrator.
Database backups that include tables or log files containing passwords should be protected using a restricted access mode.
MySQL users should use the following guidelines to keep passwords secure.
When you run a client program to connect to the MySQL server, it is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed here, along with an assessment of the risks of each method. In short, the safest methods are to have the client program prompt for the password or to specify the password in a properly protected option file.
Use a
-p or
your_pass--password=
option on the command line. For example:
your_pass
shell> mysql -u francis -pfrank db_name
This is convenient but insecure, because your password becomes visible to system status programs such as ps that may be invoked by other users to display command lines. MySQL clients typically overwrite the command-line password argument with zeros during their initialization sequence. However, there is still a brief interval during which the value is visible. Also, on some systems this overwriting strategy is ineffective and the password remains visible to ps. (SystemV Unix systems and perhaps others are subject to this problem.)
If your operating environment is set up to display your current command in the title bar of your terminal window, the password remains visible as long as the command is running, even if the command has scrolled out of view in the window content area.
Use the -p or --password
option on the command line with no password value specified.
In this case, the client program solicits the password
interactively:
shell>mysql -u francis -pEnter password:db_name********
The “*” characters indicate
where you enter your password. The password is not displayed
as you enter it.
It is more secure to enter your password this way than to specify it on the command line because it is not visible to other users. However, this method of entering a password is suitable only for programs that you run interactively. If you want to invoke a client from a script that runs noninteractively, there is no opportunity to enter the password from the keyboard. On some systems, you may even find that the first line of your script is read and interpreted (incorrectly) as your password.
Store your password in an option file. For example, on Unix
you can list your password in the
[client] section of the
.my.cnf file in your home directory:
[client] password=your_pass
To keep the password safe, the file should not be accessible
to anyone but yourself. To ensure this, set the file access
mode to 400 or 600.
For example:
shell> chmod 600 .my.cnf
To name from the command line a specific option file
containing the password, use the
--defaults-file=
option, where file_namefile_name is the full
path name to the file. For example:
shell> mysql --defaults-file=/home/francis/mysql-opts
Section 4.2.3.3, “Using Option Files”, discusses option files in more detail.
Store your password in the MYSQL_PWD
environment variable. See
Section 2.13, “Environment Variables”.
This method of specifying your MySQL password must be
considered extremely insecure and
should not be used. Some versions of ps
include an option to display the environment of running
processes. If you set MYSQL_PWD, your
password is exposed to any other user who runs
ps. Even on systems without such a
version of ps, it is unwise to assume
that there are no other methods by which users can examine
process environments.
On Unix, the mysql client writes a record of
executed statements to a history file (see
Section 4.5.1.3, “mysql Logging”). By default, this file is named
.mysql_history and is created in your home
directory. Passwords can appear as plain text in SQL statements
such as GRANT and
SET PASSWORD, so if you use these
statements, they are logged in the history file. To keep this
file safe, use a restrictive access mode, the same way as
described earlier for the .my.cnf file.
If your command interpreter is configured to maintain a history,
any file in which the commands are saved will contain MySQL
passwords entered on the command line. For example,
bash uses
~/.bash_history. Any such file should have
a restrictive access mode.
MySQL user accounts are listed in the user
table of the mysql database. Each MySQL
account is assigned a password, although what is stored in the
Password column of the
user table is not the plaintext version of
the password, but a hash value computed from it. Password hash
values are computed by the
PASSWORD() function.
MySQL uses passwords in two phases of client/server communication:
When a client attempts to connect to the server, there is an
initial authentication step in which the client must present
a password that has a hash value matching the hash value
stored in the user table for the account
that the client wants to use.
After the client connects, it can (if it has sufficient
privileges) set or change the password hashes for accounts
listed in the user table. The client can
do this by using the
PASSWORD() function to
generate a password hash, or by using the
GRANT or
SET PASSWORD statements.
In other words, the server uses hash values
during authentication when a client first attempts to connect.
The server generates hash values if a
connected client invokes the
PASSWORD() function or uses a
GRANT or SET
PASSWORD statement to set or change a password.
The password hashing mechanism was updated in MySQL 4.1 to provide better security and to reduce the risk of passwords being intercepted. However, this new mechanism is understood only by the 4.1 server and 4.1 clients, which can result in some compatibility problems. A 4.1 client can connect to a pre-4.1 server, because the client understands both the old and new password hashing mechanisms. However, a pre-4.1 client that attempts to connect to a 4.1 server may run into difficulties. For example, a 4.0 mysql client that attempts to connect to a 4.1 server may fail with the following error message:
shell> mysql -h localhost -u root
Client does not support authentication protocol requested
by server; consider upgrading MySQL client
Another common example of this phenomenon occurs for attempts to
use the older PHP mysql extension after
upgrading to MySQL 4.1 or newer. (See
Common Problems with MySQL and PHP.)
The following discussion describes the differences between the old and new password mechanisms, and what you should do if you upgrade your server to 4.1 but need to maintain backward compatibility with pre-4.1 clients. Additional information can be found in Section B.5.2.4, “Client does not support authentication protocol”. This information is of particular importance to PHP programmers migrating MySQL databases from version 4.0 or lower to version 4.1 or higher.
This discussion contrasts 4.1 behavior with pre-4.1 behavior, but the 4.1 behavior described here actually begins with 4.1.1. MySQL 4.1.0 is an “odd” release because it has a slightly different mechanism than that implemented in 4.1.1 and up. Differences between 4.1.0 and more recent versions are described further in Section 5.4.2.5, “Password Hashing in MySQL 4.1.0”.
Prior to MySQL 4.1, password hashes computed by the
PASSWORD() function are 16 bytes
long. Such hashes look like this:
mysql> SELECT PASSWORD('mypass');
+--------------------+
| PASSWORD('mypass') |
+--------------------+
| 6f8c114b58f2ce9e |
+--------------------+
The Password column of the
user table (in which these hashes are stored)
also is 16 bytes long before MySQL 4.1.
As of MySQL 4.1, the PASSWORD()
function has been modified to produce a longer 41-byte hash
value:
mysql> SELECT PASSWORD('mypass');
+-------------------------------------------+
| PASSWORD('mypass') |
+-------------------------------------------+
| *6C8989366EAF75BB670AD8EA7A7FC1176A95CEF4 |
+-------------------------------------------+
Accordingly, the Password column in the
user table also must be 41 bytes long to
store these values:
If you perform a new installation of MySQL 4.1, the
Password column is made 41 bytes long
automatically.
If you upgrade an older installation to 4.1, you should run
the mysql_fix_privilege_tables script to
increase the length of the Password
column from 16 to 41 bytes. (The script does not change
existing password values, which remain 16 bytes long.)
A widened Password column can store password
hashes in both the old and new formats. The format of any given
password hash value can be determined two ways:
The obvious difference is the length (16 bytes versus 41 bytes).
A second difference is that password hashes in the new
format always begin with a
“*” character, whereas
passwords in the old format never do.
The longer password hash format has better cryptographic properties, and client authentication based on long hashes is more secure than that based on the older short hashes.
The differences between short and long password hashes are relevant both for how the server uses passwords during authentication and for how it generates password hashes for connected clients that perform password-changing operations.
The way in which the server uses password hashes during
authentication is affected by the width of the
Password column:
If the column is short, only short-hash authentication is used.
If the column is long, it can hold either short or long hashes, and the server can use either format:
Pre-4.1 clients can connect, although because they know only about the old hashing mechanism, they can authenticate only using accounts that have short hashes.
4.1 clients can authenticate using accounts that have short or long hashes.
Even for short-hash accounts, the authentication process is actually a bit more secure for 4.1 and later clients than for older clients. In terms of security, the gradient from least to most secure is:
Pre-4.1 client authenticating with short password hash
4.1 client authenticating with short password hash
4.1 client authenticating with long password hash
The way in which the server generates password hashes for
connected clients is affected by the width of the
Password column and by the
--old-passwords option. A 4.1
server generates long hashes only if certain conditions are met:
The Password column must be wide enough to
hold long values and the
--old-passwords option must not
be given. These conditions apply as follows:
The Password column must be wide enough
to hold long hashes (41 bytes). If the column has not been
updated and still has the pre-4.1 width of 16 bytes, the
server notices that long hashes cannot fit into it and
generates only short hashes when a client performs
password-changing operations using
PASSWORD(),
GRANT, or
SET PASSWORD. This is the
behavior that occurs if you have upgraded to 4.1 but have
not yet run the
mysql_fix_privilege_tables script to
widen the Password column.
If the Password column is wide, it can
store either short or long password hashes. In this case,
PASSWORD(),
GRANT, and
SET PASSWORD generate long
hashes unless the server was started with the
--old-passwords option. That
option forces the server to generate short password hashes
instead.
The purpose of the
--old-passwords option is to
enable you to maintain backward compatibility with pre-4.1
clients under circumstances where the server would otherwise
generate long password hashes. The option does not affect
authentication (4.1 clients can still use accounts that have
long password hashes), but it does prevent creation of a long
password hash in the user table as the result
of a password-changing operation. Were that to occur, the
account no longer could be used by pre-4.1 clients. Without the
--old-passwords option, the
following undesirable scenario is possible:
An old client connects to an account that has a short password hash.
The client changes its own password. Without
--old-passwords, this results
in the account having a long password hash.
The next time the old client attempts to connect to the account, it cannot, because the account has a long password hash that requires the new hashing mechanism during authentication. (Once an account has a long password hash in the user table, only 4.1 clients can authenticate for it, because pre-4.1 clients do not understand long hashes.)
This scenario illustrates that, if you must support older
pre-4.1 clients, it is dangerous to run a 4.1 server without
using the --old-passwords option.
By running the server with
--old-passwords,
password-changing operations do not generate long password
hashes and thus do not cause accounts to become inaccessible to
older clients. (Those clients cannot inadvertently lock
themselves out by changing their password and ending up with a
long password hash.)
The downside of the
--old-passwords option is that
any passwords you create or change use short hashes, even for
4.1 clients. Thus, you lose the additional security provided by
long password hashes. If you want to create an account that has
a long hash (for example, for use by 4.1 clients), you must do
so while running the server without
--old-passwords.
The following scenarios are possible for running a 4.1 or later server:
Scenario 1: Short
Password column in user table:
Only short hashes can be stored in the
Password column.
The server uses only short hashes during client authentication.
For connected clients, password hash-generating operations
involving PASSWORD(),
GRANT, or
SET PASSWORD use short hashes
exclusively. Any change to an account's password results in
that account having a short password hash.
The --old-passwords option
can be used but is superfluous because with a short
Password column, the server generates
only short password hashes anyway.
Scenario 2: Long
Password column; server not started with
--old-passwords option:
Short or long hashes can be stored in the
Password column.
4.1 and later clients can authenticate using accounts that have short or long hashes.
Pre-4.1 clients can authenticate only using accounts that have short hashes.
For connected clients, password hash-generating operations
involving PASSWORD(),
GRANT, or
SET PASSWORD use long hashes
exclusively. A change to an account's password results in
that account having a long password hash.
As indicated earlier, a danger in this scenario is that it is
possible for accounts that have a short password hash to become
inaccessible to pre-4.1 clients. A change to such an account's
password made using GRANT,
PASSWORD(), or
SET PASSWORD results in the
account being given a long password hash. From that point on, no
pre-4.1 client can authenticate to that account until the client
upgrades to 4.1.
To deal with this problem, you can change a password in a
special way. For example, normally you use
SET PASSWORD as follows to change
an account password:
SET PASSWORD FOR 'some_user'@'some_host' = PASSWORD('mypass');
To change the password but create a short hash, use the
OLD_PASSWORD() function instead:
SET PASSWORD FOR 'some_user'@'some_host' = OLD_PASSWORD('mypass');
OLD_PASSWORD() is useful for
situations in which you explicitly want to generate a short
hash.
Scenario 3: Long
Password column; server started with
--old-passwords option:
Short or long hashes can be stored in the
Password column.
4.1 clients can authenticate for accounts that have short or
long hashes (but note that it is possible to create long
hashes only when the server is started without
--old-passwords).
Pre-4.1 clients can authenticate only for accounts that have short hashes.
For connected clients, password hash-generating operations
involving PASSWORD(),
GRANT, or
SET PASSWORD use short hashes
exclusively. Any change to an account's password results in
that account having a short password hash.
In this scenario, you cannot create accounts that have long
password hashes, because the
--old-passwords option prevents
generation of long hashes. Also, if you create an account with a
long hash before using the
--old-passwords option, changing
the account's password while
--old-passwords is in effect
results in the account being given a short password, causing it
to lose the security benefits of a longer hash.
The disadvantages for these scenarios may be summarized as follows:
In scenario 1, you cannot take advantage of longer hashes that provide more secure authentication.
In scenario 2, accounts with short hashes become inaccessible to
pre-4.1 clients if you change their passwords without explicitly
using OLD_PASSWORD().
In scenario 3, --old-passwords
prevents accounts with short hashes from becoming inaccessible,
but password-changing operations cause accounts with long hashes
to revert to short hashes, and you cannot change them back to
long hashes while --old-passwords
is in effect.
An upgrade to MySQL 4.1 can cause a compatibility issue for
applications that use PASSWORD()
to generate passwords for their own purposes. Applications
really should not do this, because
PASSWORD() should be used only to
manage passwords for MySQL accounts. But some applications use
PASSWORD() for their own purposes
anyway.
If you upgrade to 4.1 and run the server under conditions where
it generates long password hashes, an application that uses
PASSWORD() for its own passwords
breaks. The recommended course of action is to modify the
application to use another function, such as
SHA1() or
MD5(), to produce hashed values.
If that is not possible, you can use the
OLD_PASSWORD() function, which is
provided to generate short hashes in the old format. But note
that OLD_PASSWORD() may one day
no longer be supported.
If the server is running under circumstances where it generates
short hashes, OLD_PASSWORD() is
available but is equivalent to
PASSWORD().
PHP programmers migrating their MySQL databases from version 4.0 or lower to version 4.1 or higher should see MySQL and PHP.
Password hashing in MySQL 4.1.0 differs from hashing in 4.1.1 and up. The 4.1.0 differences are:
Password hashes are 45 bytes long rather than 41 bytes.
The PASSWORD() function is
nonrepeatable. That is, with a given argument
X, successive calls to
PASSWORD(
generate different results.
X)
These differences make authentication in 4.1.0 incompatible with
that of releases that follow it. If you have upgraded to MySQL
4.1.0, it is recommended that you upgrade to a newer version as
soon as possible. After you do, reassign any long passwords in
the user table so that they are compatible
with the 41-byte format.
When you connect to a MySQL server, you should use a password. The password is not transmitted in clear text over the connection. Password handling during the client connection sequence was upgraded in MySQL 4.1.1 to be very secure. If you are still using pre-4.1.1-style passwords, the encryption algorithm is not as strong as the newer algorithm. With some effort, a clever attacker who can sniff the traffic between the client and the server can crack the password. (See Section 5.4.2.3, “Password Hashing in MySQL”, for a discussion of the different password handling methods.)
All other information is transferred as text, and can be read by anyone who is able to watch the connection. If the connection between the client and the server goes through an untrusted network, and you are concerned about this, you can use the compressed protocol (in MySQL 3.22 and above) to make traffic much more difficult to decipher. You can also use MySQL's internal SSL support to make the connection even more secure in MySQL 4.0 and up. See Section 5.6.6, “Using SSL for Secure Connections”. Alternatively, use SSH to get an encrypted TCP/IP connection between a MySQL server and a MySQL client. You can find an Open Source SSH client at http://www.openssh.org/, and a commercial SSH client at http://www.ssh.com/.
To make a MySQL system secure, you should strongly consider the following suggestions:
Require all MySQL accounts to have a password. A client
program does not necessarily know the identity of the person
running it. It is common for client/server applications that
the user can specify any user name to the client program. For
example, anyone can use the mysql program
to connect as any other person simply by invoking it as
mysql -u if
other_user
db_nameother_user has no password. If all
accounts have a password, connecting using another user's
account becomes much more difficult.
For a discussion of methods for setting passwords, see Section 5.6.5, “Assigning Account Passwords”.
Make sure that the only Unix user account with read or write privileges in the database directories is the account that is used for running mysqld.
Never run the MySQL server as the Unix root
user. This is extremely dangerous, because any user with the
FILE privilege is able to cause
the server to create files as root (for
example, ~root/.bashrc). To prevent this,
mysqld refuses to run as
root unless that is specified explicitly
using the --user=root option.
mysqld can (and should) be run as an
ordinary, unprivileged user instead. You can create a separate
Unix account named mysql to make everything
even more secure. Use this account only for administering
MySQL. To start mysqld as a different Unix
user, add a user option that specifies the
user name in the [mysqld] group of the
my.cnf option file where you specify
server options. For example:
[mysqld] user=mysql
This causes the server to start as the designated user whether you start it manually or by using mysqld_safe or mysql.server. For more details, see Section 5.4.6, “How to Run MySQL as a Normal User”.
Running mysqld as a Unix user other than
root does not mean that you need to change
the root user name in the
user table. User names for MySQL
accounts have nothing to do with user names for Unix
accounts.
Do not grant the PROCESS or
SUPER privilege to
nonadministrative users. The output of mysqladmin
processlist and SHOW
PROCESSLIST shows the text of any statements
currently being executed, so any user who is permitted to see
the server process list might be able to see statements issued
by other users such as UPDATE user SET
password=PASSWORD('not_secure').
mysqld reserves an extra connection for
users who have the SUPER
privilege (PROCESS before MySQL
4.0.2), so that a MySQL root user can log
in and check server activity even if all normal connections
are in use.
The SUPER privilege can be used
to terminate client connections, change server operation by
changing the value of system variables, and control
replication servers.
Do not grant the FILE privilege
to nonadministrative users. Any user that has this privilege
can write a file anywhere in the file system with the
privileges of the mysqld daemon. To make
this a bit safer, files generated with
SELECT ... INTO
OUTFILE do not overwrite existing files and are
writable by everyone.
The FILE privilege may also be
used to read any file that is world-readable or accessible to
the Unix user that the server runs as. With this privilege,
you can read any file into a database table. This could be
abused, for example, by using LOAD
DATA to load /etc/passwd into a
table, which then can be displayed with
SELECT.
Do not permit the use of symlinks to tables. (This capability
can be disabled with the
--skip-symbolic-links
option.) This is especially important if you run
mysqld as root, because
anyone that has write access to the server's data directory
then could delete any file in the system! See
Section 7.10.2, “Using Symbolic Links for Tables on Unix”.
If you do not trust your DNS, you should use IP addresses rather than host names in the grant tables. In any case, you should be very careful about creating grant table entries using host name values that contain wildcards.
If you want to restrict the number of connections permitted to
a single account, you can do so by setting the
max_user_connections variable
in mysqld. The
GRANT statement also supports
resource control options for limiting the extent of server use
permitted to an account. See Section 12.4.1.2, “GRANT Syntax”.
If the plugin directory is writable by the server, it may be
possible for a user to write executable code to a file in the
directory using SELECT
... INTO DUMPFILE. This can be prevented by making
plugin_dir read only to the
server.
The following mysqld options affect security:
Table 5.5 Security Option/Variable Summary
| Name | Cmd-Line | Option File | System Var | Status Var | Var Scope | Dynamic |
|---|---|---|---|---|---|---|
| allow-suspicious-udfs | Yes | Yes | ||||
| chroot | Yes | Yes | ||||
| des-key-file | Yes | Yes | ||||
| local_infile | Yes | Global | Yes | |||
| old_passwords | Yes | Both | Yes | |||
| safe-show-database | Yes | Yes | Global | Yes | ||
| - Variable: safe_show_database | Yes | Global | Yes | |||
| safe-user-create | Yes | Yes | ||||
| secure-auth | Yes | Yes | Global | Yes | ||
| - Variable: secure_auth | Yes | Global | Yes | |||
| skip-grant-tables | Yes | Yes | ||||
| skip-name-resolve | Yes | Yes | ||||
| - Variable: skip_name_resolve | ||||||
| skip-networking | Yes | Yes | Global | No | ||
| - Variable: skip_networking | Yes | Global | No | |||
| skip-show-database | Yes | Yes | Global | No | ||
| - Variable: skip_show_database | Yes | Global | No |
This option controls whether user-defined functions that have
only an xxx symbol for the main function
can be loaded. By default, the option is turned off and only
UDFs that have at least one auxiliary symbol can be loaded;
this prevents attempts at loading functions from shared object
files other than those containing legitimate UDFs. This option
was added in MySQL 4.0.24 and 4.1.10a. See
Section 18.2.2.6, “User-Defined Function Security Precautions”.
If you start the server with
--local-infile=0, clients
cannot use LOCAL in
LOAD DATA statements. See
Section 5.4.5, “Security Issues with LOAD DATA LOCAL”.
Force the server to generate short (pre-4.1) password hashes for new passwords. This is useful for compatibility when the server must support older client programs. See Section 5.4.2.3, “Password Hashing in MySQL”.
If this option is enabled, a user cannot create new MySQL
users by using the GRANT
statement unless the user has the
INSERT privilege for the
mysql.user table. If you want a user to
have the ability to create new users that have those
privileges that the user has the right to grant, you should
grant the user the following privilege:
GRANT INSERT(user) ON mysql.user TO 'user_name'@'host_name';
This ensures that the user cannot change any privilege columns
directly, but has to use the
GRANT statement to give
privileges to other users.
Disallow authentication for accounts that have old (pre-4.1) passwords. This option is available as of MySQL 4.1.1.
The mysql client also has a
--secure-auth option, which
prevents connections to a server if the server requires a
password in old format for the client account.
This option causes the server not to use the privilege system
at all. This gives anyone with access to the server
unrestricted access to all
databases. You can cause a running server to start
using the grant tables again by executing mysqladmin
flush-privileges or mysqladmin
reload command from a system shell, or by issuing a
MySQL FLUSH
PRIVILEGES statement. This option also suppresses
loading of user-defined functions (UDFs).
Host names are not resolved. All Host
column values in the grant tables must be IP addresses or
localhost.
Do not permit TCP/IP connections over the network. All connections to mysqld must be made using Unix socket files. This option is unsuitable when using a MySQL version prior to 3.23.27 with the MIT-pthreads package, because Unix socket files were not supported by MIT-pthreads at that time.
With this option, the SHOW
DATABASES statement is permitted only to users who
have the SHOW DATABASES
privilege, and the statement displays all database names.
Without this option, SHOW
DATABASES is permitted to all users, but displays
each database name only if the user has the
SHOW DATABASES privilege or
some privilege for the database. Note that any global
privilege is a privilege for the database.
Options that begin with --ssl
specify whether to permit clients to connect using SSL and
indicate where to find SSL keys and certificates. See
Section 5.6.6.3, “SSL Command Options”.
The LOAD DATA statement can load a
file that is located on the server host, or it can load a file
that is located on the client host when the
LOCAL keyword is specified.
There are two potential security issues with supporting the
LOCAL version of LOAD
DATA statements:
The transfer of the file from the client host to the server
host is initiated by the MySQL server. In theory, a patched
server could be built that would tell the client program to
transfer a file of the server's choosing rather than the file
named by the client in the LOAD
DATA statement. Such a server could access any file
on the client host to which the client user has read access.
In a Web environment where the clients are connecting from a
Web server, a user could use
LOAD DATA
LOCAL to read any files that the Web server process
has read access to (assuming that a user could run any command
against the SQL server). In this environment, the client with
respect to the MySQL server actually is the Web server, not
the remote program being run by the user who connects to the
Web server.
To deal with these problems, we changed how
LOAD DATA
LOCAL is handled as of MySQL 3.23.49 and MySQL 4.0.2
(4.0.13 on Windows):
By default, all MySQL clients and libraries in binary
distributions are compiled with the
--enable-local-infile option, to be
compatible with MySQL 3.23.48 and before.
If you build MySQL from source but do not invoke
configure with the
--enable-local-infile option,
LOAD DATA
LOCAL cannot be used by any client unless it is
written explicitly to invoke
mysql_options(...
MYSQL_OPT_LOCAL_INFILE, 0). See
Section 17.6.6.47, “mysql_options()”.
You can disable all
LOAD DATA
LOCAL statements from the server side by starting
mysqld with the
--local-infile=0 option.
For the mysql command-line client, enable
LOAD DATA
LOCAL by specifying the
--local-infile[=1] option, or
disable it with the
--local-infile=0 option. For
mysqlimport, local data file loading is off
by default; enable it with the
--local or
-L option. In any case, successful use of a
local load operation requires that the server permits it.
If you use LOAD
DATA LOCAL in Perl scripts or other programs that
read the [client] group from option files,
you can add the local-infile=1 option to
that group. However, to keep this from causing problems for
programs that do not understand
local-infile, specify it using the
loose- prefix:
[client] loose-local-infile=1
The loose- prefix can be used as of MySQL
4.0.2.
If LOAD DATA
LOCAL is disabled, either in the server or the
client, a client that attempts to issue such a statement
receives the following error message:
ERROR 1148: The used command is not allowed with this MySQL version
On Windows, you can run the server as a Windows service using a normal user account beginning with MySQL 4.0.17 and 4.1.2. (Older MySQL versions required you to have administrator rights. This was a bug introduced in MySQL 3.23.54.)
On Unix, the MySQL server mysqld can be started
and run by any user. However, you should avoid running the server
as the Unix root user for security reasons. To
change mysqld to run as a normal unprivileged
Unix user user_name, you must do the
following:
Stop the server if it is running (use mysqladmin shutdown).
Change the database directories and files so that
user_name has privileges to read
and write files in them (you might need to do this as the Unix
root user):
shell> chown -R user_name /path/to/mysql/datadir
If you do not do this, the server will not be able to access
databases or tables when it runs as
user_name.
If directories or files within the MySQL data directory are
symbolic links, chown -R might not follow
symbolic links for you. If it does not, you will also need to
follow those links and change the directories and files they
point to.
Start the server as user user_name.
Another alternative is to start mysqld as
the Unix root user and use the
--user=
option. mysqld starts up, then switches to
run as the Unix user user_nameuser_name
before accepting any connections.
To start the server as the given user automatically at system
startup time, specify the user name by adding a
user option to the
[mysqld] group of the
/etc/my.cnf option file or the
my.cnf option file in the server's data
directory. For example:
[mysqld]
user=user_name
If your Unix machine itself is not secured, you should assign
passwords to the MySQL root accounts in the
grant tables. Otherwise, any user with a login account on that
machine can run the mysql client with a
--user=root option and perform any
operation. (It is a good idea to assign passwords to MySQL
accounts in any case, but especially so when other login accounts
exist on the server host.) See Section 2.10, “Postinstallation Setup and Testing”.
The primary function of the MySQL privilege system is to
authenticate a user who connects from a given host and to associate
that user with privileges on a database such as
SELECT,
INSERT,
UPDATE, and
DELETE. Additional functionality
includes the ability to have anonymous users and to grant privileges
for MySQL-specific functions such as
LOAD DATA
INFILE and administrative operations.
There are some things that you cannot do with the MySQL privilege system:
You cannot explicitly specify that a given user should be denied access. That is, you cannot explicitly match a user and then refuse the connection.
You cannot specify that a user has privileges to create or drop tables in a database but not to create or drop the database itself.
A password applies globally to an account. You cannot associate a password with a specific object such as a database or table.
The user interface to the MySQL privilege system consists of SQL
statements such as GRANT and
REVOKE. See
Section 12.4.1, “Account Management Statements”.
Internally, the server stores privilege information in the grant
tables of the mysql database (that is, in the
database named mysql). The MySQL server reads the
contents of these tables into memory when it starts and bases
access-control decisions on the in-memory copies of the grant
tables.
The MySQL privilege system ensures that all users may perform only the operations permitted to them. As a user, when you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. When you issue requests after connecting, the system grants privileges according to your identity and what you want to do.
MySQL considers both your host name and user name in identifying you
because there is no reason to assume that a given user name belongs
to the same person on all hosts. For example, the user
joe who connects from
office.example.com need not be the same person as
the user joe who connects from
home.example.com. MySQL handles this by enabling
you to distinguish users on different hosts that happen to have the
same name: You can grant one set of privileges for connections by
joe from office.example.com,
and a different set of privileges for connections by
joe from home.example.com. To
see what privileges a given account has, use the
SHOW GRANTS statement. For example:
SHOW GRANTS FOR 'joe'@'office.example.com'; SHOW GRANTS FOR 'joe'@'home.example.com';
MySQL access control involves two stages when you run a client program that connects to the server:
Stage 1: The server accepts or rejects the connection based on your identity and whether you can verify your identity by supplying the correct password.
Stage 2: Assuming that you can
connect, the server checks each statement you issue to determine
whether you have sufficient privileges to perform it. For example,
if you try to select rows from a table in a database or drop a table
from the database, the server verifies that you have the
SELECT privilege for the table or the
DROP privilege for the database.
For a more detailed description of what happens during each stage, see Section 5.5.4, “Access Control, Stage 1: Connection Verification”, and Section 5.5.5, “Access Control, Stage 2: Request Verification”.
If your privileges are changed (either by yourself or someone else) while you are connected, those changes do not necessarily take effect immediately for the next statement that you issue. For details about the conditions under which the server reloads the grant tables, see Section 5.5.6, “When Privilege Changes Take Effect”.
For general security-related advice, see Section 5.4, “General Security Issues”. For help in diagnosing privilege-related problems, see Section 5.5.7, “Causes of Access-Denied Errors”.
MySQL provides privileges that apply in different contexts and at different levels of operation:
Administrative privileges enable users to manage operation of the MySQL server. These privileges are global because they are not specific to a particular database.
Database privileges apply to a database and to all objects within it. These privileges can be granted for specific databases, or globally so that they apply to all databases.
Privileges for database objects such as tables, indexes, views, and stored routines can be granted for specific objects within a database, for all objects of a given type within a database (for example, all tables in a database), or globally for all objects of a given type in all databases).
Information about account privileges is stored in the
user, db,
host, tables_priv, and
columns_priv tables in the
mysql database (see
Section 5.5.2, “Privilege System Grant Tables”). The MySQL server reads
the contents of these tables into memory when it starts and
reloads them under the circumstances indicated in
Section 5.5.6, “When Privilege Changes Take Effect”. Access-control decisions are
based on the in-memory copies of the grant tables.
Some releases of MySQL introduce changes to the structure of the grant tables to add new access privileges or features. Whenever you update to a new version of MySQL, you should update your grant tables to make sure that they have the current structure so that you can take advantage of any new capabilities. See Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.
The following table shows the privilege names used at the SQL
level in the GRANT and
REVOKE statements, along with the
column name associated with each privilege in the grant tables and
the context in which the privilege applies.
Table 5.6 Permissible Privileges for GRANT and REVOKE
| Privilege | Column | Context |
|---|---|---|
CREATE | Create_priv | databases, tables, or indexes |
DROP | Drop_priv | databases or tables |
GRANT OPTION | Grant_priv | databases, tables, or stored routines |
REFERENCES | References_priv | databases or tables |
ALTER | Alter_priv | tables |
DELETE | Delete_priv | tables |
INDEX | Index_priv | tables |
INSERT | Insert_priv | tables or columns |
SELECT | Select_priv | tables or columns |
UPDATE | Update_priv | tables or columns |
CREATE TEMPORARY TABLES | Create_tmp_table_priv | tables |
LOCK TABLES | Lock_tables_priv | tables |
FILE | File_priv | file access on server host |
PROCESS | Process_priv | server administration |
RELOAD | Reload_priv | server administration |
REPLICATION CLIENT | Repl_client_priv | server administration |
REPLICATION SLAVE | Repl_slave_priv | server administration |
SHOW DATABASES | Show_db_priv | server administration |
SHUTDOWN | Shutdown_priv | server administration |
SUPER | Super_priv | server administration |
ALL [PRIVILEGES] | server administration | |
USAGE | server administration |
The following list provides a general description of each privilege available in MySQL. Particular SQL statements might have more specific privilege requirements than indicated here. If so, the description for the statement in question provides the details.
The ALL or
ALL PRIVILEGES
privilege specifier is shorthand. It stands for “all
privileges available at a given privilege level”
(except GRANT OPTION). For
example, granting ALL at the
global or table level grants all global privileges or all
table-level privileges.
The ALTER privilege enables use
of ALTER TABLE to change the
structure of or rename tables. (ALTER
TABLE also requires the
INSERT and
CREATE privileges.)
The CREATE privilege enables
creation of new databases and tables.
The CREATE TEMPORARY TABLES
privilege enables the use of the keyword
TEMPORARY in CREATE
TABLE statements. This privilege was added in MySQL
4.0.2.
The DELETE privilege enables
rows to be deleted from tables in a database.
The DROP privilege enables you
to drop (remove) existing databases and tables. If you grant
the DROP privilege for the
mysql database to a user, that user can
drop the database in which the MySQL access privileges are
stored!
The EXECUTE privilege was added
in MySQL 4.0.2, but is not used until MySQL 5.0.
The FILE privilege gives you
permission to read and write files on the server host using
the LOAD DATA
INFILE and
SELECT ... INTO
OUTFILE statements and the
LOAD_FILE() function. A user
who has the FILE privilege can
read any file on the server host that is either world-readable
or readable by the MySQL server. (This implies the user can
read any file in any database directory, because the server
can access any of those files.) The
FILE privilege also enables the
user to create new files in any directory where the MySQL
server has write access. This includes the server's data
directory containing the files that implement the privilege
tables. As a security measure, the server will not overwrite
existing files.
The GRANT OPTION privilege
enables you to give to other users or remove from other users
those privileges that you yourself possess.
The INDEX privilege enables you
to create or drop (remove) indexes.
INDEX applies to existing
tables. If you have the CREATE
privilege for a table, you can include index definitions in
the CREATE TABLE statement.
The INSERT privilege enables
rows to be inserted into tables in a database.
INSERT is also required for the
ANALYZE TABLE,
OPTIMIZE TABLE, and
REPAIR TABLE table-maintenance
statements.
The LOCK TABLES privilege
enables the use of explicit LOCK
TABLES statements to lock tables for which you have
the SELECT privilege. This
includes the use of write locks, which prevents other sessions
from reading the locked table. This privilege was added in
MySQL 4.0.2.
The PROCESS privilege pertains
to display of information about the threads executing within
the server (that is, information about the statements being
executed by sessions). The privilege enables use of
SHOW PROCESSLIST or
mysqladmin processlist to see threads
belonging to other accounts; you can always see your own
threads. Prior to MySQL 4.0.2,
PROCESS also enable the use of
KILL to kill threads belonging
to other accounts; you can always kill your own threads.
The REFERENCES privilege
currently is unused.
The RELOAD privilege enables
use of the FLUSH statement. It
also enables mysqladmin commands that are
equivalent to FLUSH operations:
flush-hosts, flush-logs,
flush-privileges,
flush-status,
flush-tables,
flush-threads, refresh,
and reload.
The reload command tells the server to
reload the grant tables into memory.
flush-privileges is a synonym for
reload. The refresh
command closes and reopens the log files and flushes all
tables. The other
flush-
commands perform functions similar to
xxxrefresh, but are more specific and may be
preferable in some instances. For example, if you want to
flush just the log files, flush-logs is a
better choice than refresh.
The REPLICATION CLIENT
privilege enables the use of SHOW MASTER
STATUS and SHOW SLAVE
STATUS. This privilege was added in MySQL 4.0.2.
The REPLICATION SLAVE privilege
should be granted to accounts that are used by slave servers
to connect to the current server as their master. Without this
privilege, the slave cannot request updates that have been
made to databases on the master server. This privilege was
added in MySQL 4.0.2.
The SELECT privilege enables
you to select rows from tables in a database.
SELECT statements require the
SELECT privilege only if they
actually retrieve rows from a table. Some
SELECT statements do not access
tables and can be executed without permission for any
database. For example, you can use
SELECT as a simple calculator
to evaluate expressions that make no reference to tables:
SELECT 1+1; SELECT PI()*2;
The SELECT privilege is also
needed for other statements that read column values. For
example, SELECT is needed for
columns referenced on the right hand side of
col_name=expr
assignment in UPDATE statements
or for columns named in the WHERE clause of
DELETE or
UPDATE statements.
The SHOW DATABASES privilege
enables the account to see database names by issuing the
SHOW DATABASE statement. Accounts that do
not have this privilege see only databases for which they have
some privileges, and cannot use the statement at all if the
server was started with the
--skip-show-database option.
Note that any global privilege is a
privilege for the database. SHOW
DATABASES was added in MySQL 4.0.2.
The SHUTDOWN privilege enables
use of the mysqladmin shutdown command.
There is no corresponding SQL statement.
The SUPER privilege enables an
account to use CHANGE MASTER
TO, KILL or
mysqladmin kill to kill threads belonging
to other accounts (you can always kill your own threads),
PURGE BINARY LOGS,
configuration changes using
SET
GLOBAL to modify global system variables, the
mysqladmin debug command, enabling or
disabling logging, performing updates even if the
read_only system variable is
enabled, starting and stopping replication on slave servers,
and enables you to connect (once) even if the connection limit
controlled by the
max_connections system
variable is reached. This privilege was added in MySQL 4.0.2.
Prior to MySQL 4.0.2, the
PROCESS privilege controls the
ability to terminate threads for other accounts.
The UPDATE privilege enables
rows to be updated in tables in a database.
The USAGE privilege specifier
stands for “no privileges.” It is used at the
global level with GRANT to
modify account attributes such as resource limits or SSL
characteristics without affecting existing account privileges.
It is a good idea to grant to an account only those privileges
that it needs. You should exercise particular caution in granting
the FILE and administrative
privileges:
The FILE privilege can be
abused to read into a database table any files that the MySQL
server can read on the server host. This includes all
world-readable files and files in the server's data directory.
The table can then be accessed using
SELECT to transfer its contents
to the client host.
The GRANT OPTION privilege
enables users to give their privileges to other users. Two
users that have different privileges and with the
GRANT OPTION privilege are able
to combine privileges.
The ALTER privilege may be used
to subvert the privilege system by renaming tables.
The SHUTDOWN privilege can be
abused to deny service to other users entirely by terminating
the server.
The PROCESS privilege can be
used to view the plain text of currently executing statements,
including statements that set or change passwords.
The SUPER privilege can be used
to terminate other sessions or change how the server operates.
Privileges granted for the mysql database
itself can be used to change passwords and other access
privilege information. Passwords are stored encrypted, so a
malicious user cannot simply read them to know the plain text
password. However, a user with write access to the
user table Password
column can change an account's password, and then connect to
the MySQL server using that account.
Normally, you manipulate the contents of the grant tables in the
mysql database indirectly by using statements
such as GRANT and
REVOKE to set up accounts and
control the privileges available to each one. See
Section 12.4.1, “Account Management Statements”. The discussion here
describes the underlying structure of the grant tables and how the
server uses their contents when interacting with clients.
These mysql database tables contain grant
information:
user: Contains user accounts, global
privileges, and other non-privilege columns.
db: Contains database-level privileges.
host: Obsolete.
tables_priv: Contains table-level
privileges.
columns_priv: Contains column-level
privileges.
Other tables in the mysql database do not hold
grant information and are discussed elsewhere:
func: Contains information about
user-defined functions: See
Section 18.2, “Adding New Functions to MySQL”.
help_: These
tables are used for server-side help: See
Section 5.1.7, “Server-Side Help”.
xxx
time_zone_:
These tables contain time zone information: See
Section 9.7, “MySQL Server Time Zone Support”.
xxx
Each grant table contains scope columns and privilege columns:
Scope columns determine the scope of each row (entry) in the
tables; that is, the context in which the row applies. For
example, a user table row with
Host and User values of
'thomas.loc.gov' and
'bob' would be used for authenticating
connections made to the server from the host
thomas.loc.gov by a client that specifies a
user name of bob. Similarly, a
db table row with Host,
User, and Db column
values of 'thomas.loc.gov',
'bob' and 'reports'
would be used when bob connects from the
host thomas.loc.gov to access the
reports database. The
tables_priv and
columns_priv tables contain scope columns
indicating tables or table/column combinations to which each
row applies.
Privilege columns indicate which privileges are granted by a table row; that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. Section 5.5.5, “Access Control, Stage 2: Request Verification”, describes the rules that are used to do this.
The server uses the grant tables in the following manner:
The user table scope columns determine
whether to reject or permit incoming connections. For
permitted connections, any privileges granted in the
user table indicate the user's global
privileges. Any privilege granted in this table applies to
all databases on the server.
Because any global privilege is considered a privilege for
all databases, any global privilege enables a user to see
all database names with SHOW
DATABASES.
The db table scope columns determine which
users can access which databases from which hosts. The
privilege columns determine which operations are permitted. A
privilege granted at the database level applies to the
database and to all objects in the database, such as tables
and stored programs.
The host table is used in conjunction with
the db table when you want a given
db table row to apply to several hosts. For
example, if you want a user to be able to use a database from
several hosts in your network, leave the
Host value empty in the user's
db table row, then populate the
host table with a row for each of those
hosts. This mechanism is described more detail in
Section 5.5.5, “Access Control, Stage 2: Request Verification”.
The tables_priv and
columns_priv tables are similar to the
db table, but are more fine-grained: They
apply at the table and column levels rather than at the
database level. A privilege granted at the table level applies
to the table and to all its columns. A privilege granted at
the column level applies only to a specific column.
The server uses the user,
db, and host tables in the
mysql database at both the first and second
stages of access control (see Section 5.5, “The MySQL Access Privilege System”).
The columns in the user and
db tables are shown here. The
host table is similar to the
db table but has a specialized use as described
in Section 5.5.5, “Access Control, Stage 2: Request Verification”.
Table 5.7 user and db Table Columns
| Table Name | user | db |
|---|---|---|
| Scope columns | Host | Host |
User | Db | |
Password | User | |
| Privilege columns | Select_priv | Select_priv |
Insert_priv | Insert_priv | |
Update_priv | Update_priv | |
Delete_priv | Delete_priv | |
Index_priv | Index_priv | |
Alter_priv | Alter_priv | |
Create_priv | Create_priv | |
Drop_priv | Drop_priv | |
Grant_priv | Grant_priv | |
References_priv | References_priv | |
Execute_priv | ||
Reload_priv | ||
Shutdown_priv | ||
Process_priv | ||
File_priv | ||
Show_db_priv | ||
Super_priv | ||
Create_tmp_table_priv | Create_tmp_table_priv | |
Lock_tables_priv | Lock_tables_priv | |
Repl_slave_priv | ||
Repl_client_priv | ||
| Security columns | ssl_type | |
ssl_cipher | ||
x509_issuer | ||
x509_subject | ||
| Resource control columns | max_questions | |
max_updates | ||
max_connections | ||
max_user_connections |
The ssl_type, ssl_cipher,
x509_issuer, and
x509_subject columns were added in MySQL 4.0.0.
The Create_tmp_table_priv,
Execute_priv,
Lock_tables_priv,
Repl_client_priv,
Repl_slave_priv,
Show_db_priv, Super_priv,
max_questions, max_updates,
and max_connections columns were added in MySQL
4.0.2. Execute_priv is not operational through
MySQL 4.1.
During the second stage of access control, the server performs
request verification to make sure that each client has sufficient
privileges for each request that it issues. In addition to the
user, db, and
host grant tables, the server may also consult
the tables_priv and
columns_priv tables for requests that involve
tables. The latter tables provide finer privilege control at the
table and column levels. They have the columns shown in the
following table.
Table 5.8 tables_priv and columns_priv Table Columns
| Table Name | tables_priv | columns_priv |
|---|---|---|
| Scope columns | Host | Host |
Db | Db | |
User | User | |
Table_name | Table_name | |
Column_name | ||
| Privilege columns | Table_priv | Column_priv |
Column_priv | ||
| Other columns | Timestamp | Timestamp |
Grantor |
The Timestamp and Grantor
columns are set to the current timestamp and the
CURRENT_USER value, respectively.
However, they are unused and are discussed no further here.
Scope columns in the grant tables contain strings. They are declared as shown here; the default value for each is the empty string.
Table 5.9 Grant Table Scope Column Types
| Column Name | Type |
|---|---|
| Column Name | Type |
Host | CHAR(60) |
User | CHAR(16) |
Password | CHAR(41) |
Db | CHAR(64) |
Table_name | CHAR(64) |
Column_name | CHAR(64) |
Routine_name | CHAR(64) |
Before MySQL 3.23, the Db column is
CHAR(32) in some tables and
CHAR(60) in others.
For access-checking purposes, comparisons of
User, Password,
Db, and Table_name values
are case sensitive. Comparisons of Host values
are not case sensitive. Comparisons of
Column_name values are not case sensitive as of
MySQL 3.22.12.
In the user, db, and
host tables, each privilege is listed in a
separate column that is declared as ENUM('N','Y') DEFAULT
'N'. In other words, each privilege can be disabled or
enabled, with the default being disabled.
In the tables_priv and
columns_priv tables, the privilege columns are
declared as SET columns. Values in
these columns can contain any combination of the privileges
controlled by the table. Only those privileges listed in the
column value are enabled.
Table 5.10 Set-Type Privilege Column Values
| Table Name | Column Name | Possible Set Elements |
|---|---|---|
| Table Name | Column Name | Possible Set Elements |
tables_priv | Table_priv | 'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop',
'Grant', 'References', 'Index', 'Alter' |
tables_priv | Column_priv | 'Select', 'Insert', 'Update', 'References' |
columns_priv | Column_priv | 'Select', 'Insert', 'Update', 'References' |
Administrative privileges (such as
RELOAD or
SHUTDOWN) are specified only in the
user table. Administrative operations are
operations on the server itself and are not database-specific, so
there is no reason to list these privileges in the other grant
tables. Consequently, to determine whether you can perform an
administrative operation, the server need consult only the
user table.
The FILE privilege also is
specified only in the user table. It is not an
administrative privilege as such, but your ability to read or
write files on the server host is independent of the database you
are accessing.
The mysqld server reads the contents of the
grant tables into memory when it starts. You can tell it to reload
the tables by issuing a
FLUSH PRIVILEGES
statement or executing a mysqladmin
flush-privileges or mysqladmin reload
command. Changes to the grant tables take effect as indicated in
Section 5.5.6, “When Privilege Changes Take Effect”.
When you modify an account's privileges, it is a good idea to
verify that the changes set up privileges the way you want. To
check the privileges for a given account, use the
SHOW GRANTS statement (see
Section 12.4.5.12, “SHOW GRANTS Syntax”). For example, to determine the
privileges that are granted to an account with user name and host
name values of bob and
pc84.example.com, use this statement:
SHOW GRANTS FOR 'bob'@'pc84.example.com';
MySQL account names consist of a user name and a host name. This enables creation of accounts for users with the same name who can connect from different hosts. This section describes how to write account names, including special values and wildcard rules.
In SQL statements such as GRANT and
SET PASSWORD, write account names
using the following rules:
Syntax for account names is
'.
user_name'@'host_name'
An account name consisting only of a user name is equivalent
to
'.
For example, user_name'@'%''me' is equivalent to
'me'@'%'.
The user name and host name need not be quoted if they are
legal as unquoted identifiers. Quotes are necessary to specify
a user_name string containing
special characters (such as
“-”), or a
host_name string containing special
characters or wildcard characters (such as
“%”); for example,
'test-user'@'%.com'.
Quote user names and host names as identifiers or as strings,
using either backticks (“`”),
single quotation marks (“'”),
or double quotation marks
(“"”).
The user name and host name parts, if quoted, must be quoted
separately. That is, write
'me'@'localhost', not
'me@localhost'; the latter is interpreted
as 'me@localhost'@'%'.
MySQL stores account names in grant tables in the
mysql database using separate columns for the
user name and host name parts:
The user table contains one row for each
account. The User and
Host columns store the user name and host
name. This table also indicates which global privileges the
account has.
Other grant tables indicate privileges an account has for
databases and objects within databases. These tables have
User and Host columns to
store the account name. Each row in these tables associates
with the account in the user table that has
the same User and Host
values.
For additional detail about grant table structure, see Section 5.5.2, “Privilege System Grant Tables”.
User names and host names have certain special values or wildcard conventions, as described following.
A user name is either a nonblank value that literally matches the
user name for incoming connection attempts, or a blank value
(empty string) that matches any user name. An account with a blank
user name is an anonymous user. To specify an anonymous user in
SQL statements, use a quoted empty user name part, such as
''@'localhost'.
The host name part of an account name can take many forms, and wildcards are permitted:
A host value can be a host name or an IP address. The name
'localhost' indicates the local host. The
IP address '127.0.0.1' indicates the
loopback interface.
You can use the wildcard characters
“%” and
“_” in host values. These have
the same meaning as for pattern-matching operations performed
with the LIKE operator. For
example, a host value of '%' matches any
host name, whereas a value of '%.mysql.com'
matches any host in the mysql.com domain.
'192.168.1.%' matches any host in the
192.168.1 class C network.
Because you can use IP wildcard values in host values (for
example, '192.168.1.%' to match every host
on a subnet), someone could try to exploit this capability by
naming a host 192.168.1.somewhere.com. To
foil such attempts, MySQL disallows matching on host names
that start with digits and a dot. Thus, if you have a host
named something like 1.2.example.com, its
name never matches the host part of account names. An IP
wildcard value can match only IP addresses, not host names.
For a host value specified as an IP address, you can specify a
netmask indicating how many address bits to use for the
network number. The syntax is
host_ip/netmask
GRANT ALL PRIVILEGES ON db.* TO 'david'@'192.58.197.0/255.255.255.0';
This enables david to connect from any
client host having an IP address
client_ip for which the following
condition is true:
client_ip&netmask=host_ip
That is, for the GRANT
statement just shown:
client_ip & 255.255.255.0 = 192.58.197.0
IP addresses that satisfy this condition and can connect to
the MySQL server are those in the range from
192.58.197.0 to
192.58.197.255.
The netmask can only be used to tell the server to use 8, 16, 24, or 32 bits of the address. Examples:
192.0.0.0/255.0.0.0: Any host on the
192 class A network
192.168.0.0/255.255.0.0: Any host on
the 192.168 class B network
192.168.1.0/255.255.255.0: Any host on
the 192.168.1 class C network
192.168.1.1: Only the host with this
specific IP address
The following netmask will not work because it masks 28 bits, and 28 is not a multiple of 8:
192.168.0.1/255.255.255.240
When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, and then enters Stage 2 and waits for requests.
Your identity is based on two pieces of information:
The client host from which you connect
Your MySQL user name
A reference to the
CURRENT_USER() (or
CURRENT_USER) function is
equivalent to specifying the current user's name and host name
literally.
Identity checking is performed using the three
user table scope columns
(Host, User, and
Password). The server accepts the connection
only if the Host and User
columns in some user table row match the client
host name and user name and the client supplies the password
specified in that row. The rules for permissible
Host and User values are
given in Section 5.5.3, “Specifying Account Names”.
If the User column value is nonblank, the user
name in an incoming connection must match exactly. If the
User value is blank, it matches any user name.
If the user table row that matches an incoming
connection has a blank user name, the user is considered to be an
anonymous user with no name, not a user with the name that the
client actually specified. This means that a blank user name is
used for all further access checking for the duration of the
connection (that is, during Stage 2).
The Password column can be blank. This is not a
wildcard and does not mean that any password matches. It means
that the user must connect without specifying a password.
Nonblank Password values in the
user table represent encrypted passwords. MySQL
does not store passwords in plaintext form for anyone to see.
Rather, the password supplied by a user who is attempting to
connect is encrypted (using the
PASSWORD() function). The encrypted
password then is used during the connection process when checking
whether the password is correct. (This is done without the
encrypted password ever traveling over the connection.) See
Section 5.6.1, “User Names and Passwords”.
From MySQL's point of view, the encrypted password is the
real password, so you should never give
anyone access to it. In particular, do not give
nonadministrative users read access to tables in the
mysql database.
The following table shows how various combinations of
Host and User values in the
user table apply to incoming connections.
Host Value | User Value | Permissible Connections |
|---|---|---|
'thomas.loc.gov' | 'fred' | fred, connecting from
thomas.loc.gov |
'thomas.loc.gov' | '' | Any user, connecting from thomas.loc.gov |
'%' | 'fred' | fred, connecting from any host |
'%' | '' | Any user, connecting from any host |
'%.loc.gov' | 'fred' | fred, connecting from any host in the
loc.gov domain |
'x.y.%' | 'fred' | fred, connecting from x.y.net,
x.y.com, x.y.edu,
and so on; this is probably not useful |
'144.155.166.177' | 'fred' | fred, connecting from the host with IP address
144.155.166.177 |
'144.155.166.%' | 'fred' | fred, connecting from any host in the
144.155.166 class C subnet |
'144.155.166.0/255.255.255.0' | 'fred' | Same as previous example |
It is possible for the client host name and user name of an
incoming connection to match more than one row in the
user table. The preceding set of examples
demonstrates this: Several of the entries shown match a connection
from thomas.loc.gov by fred.
When multiple matches are possible, the server must determine which of them to use. It resolves this issue as follows:
Whenever the server reads the user table
into memory, it sorts the rows.
When a client attempts to connect, the server looks through the rows in sorted order.
The server uses the first row that matches the client host name and user name.
To see how this works, suppose that the user
table looks like this:
+-----------+----------+- | Host | User | ... +-----------+----------+- | % | root | ... | % | jeffrey | ... | localhost | root | ... | localhost | | ... +-----------+----------+-
When the server reads the table into memory, it orders the rows
with the most-specific Host values first.
Literal host names and IP addresses are the most specific. (The
specificity of a literal IP address is not affected by whether it
has a netmask, so 192.168.1.13 and
192.168.1.0/255.255.255.0 are considered
equally specific.) The pattern '%' means
“any host” and is least specific. Rows with the same
Host value are ordered with the most-specific
User values first (a blank
User value means “any user” and is
least specific). For the user table just shown,
the result after sorting looks like this:
+-----------+----------+- | Host | User | ... +-----------+----------+- | localhost | root | ... | localhost | | ... | % | jeffrey | ... | % | root | ... +-----------+----------+-
When a client attempts to connect, the server looks through the
sorted rows and uses the first match found. For a connection from
localhost by jeffrey, two of
the rows from the table match: the one with
Host and User values of
'localhost' and '', and the
one with values of '%' and
'jeffrey'. The 'localhost'
row appears first in sorted order, so that is the one the server
uses.
Here is another example. Suppose that the user
table looks like this:
+----------------+----------+- | Host | User | ... +----------------+----------+- | % | jeffrey | ... | thomas.loc.gov | | ... +----------------+----------+-
The sorted table looks like this:
+----------------+----------+- | Host | User | ... +----------------+----------+- | thomas.loc.gov | | ... | % | jeffrey | ... +----------------+----------+-
A connection by jeffrey from
thomas.loc.gov is matched by the first row,
whereas a connection by jeffrey from any host
is matched by the second.
It is a common misconception to think that, for a given user
name, all rows that explicitly name that user are used first
when the server attempts to find a match for the connection.
This is not true. The preceding example illustrates this, where
a connection from thomas.loc.gov by
jeffrey is first matched not by the row
containing 'jeffrey' as the
User column value, but by the row with no
user name. As a result, jeffrey is
authenticated as an anonymous user, even though he specified a
user name when connecting.
If you are able to connect to the server, but your privileges are
not what you expect, you probably are being authenticated as some
other account. To find out what account the server used to
authenticate you, use the
CURRENT_USER() function. (See
Section 11.13, “Information Functions”.) It returns a value in
user_name@host_nameUser and
Host values from the matching
user table row. Suppose that
jeffrey connects and issues the following
query:
mysql> SELECT CURRENT_USER();
+----------------+
| CURRENT_USER() |
+----------------+
| @localhost |
+----------------+
The result shown here indicates that the matching
user table row had a blank
User column value. In other words, the server
is treating jeffrey as an anonymous user.
The CURRENT_USER() function is
available as of MySQL 4.0.6. See
Section 11.13, “Information Functions”. Another way to diagnose
authentication problems is to print out the
user table and sort it by hand to see where the
first match is being made.
After you establish a connection, the server enters Stage 2 of
access control. For each request that you issue through that
connection, the server determines what operation you want to
perform, then checks whether you have sufficient privileges to do
so. This is where the privilege columns in the grant tables come
into play. These privileges can come from any of the
user, db,
host, tables_priv, or
columns_priv tables. (You may find it helpful
to refer to Section 5.5.2, “Privilege System Grant Tables”, which lists
the columns present in each of the grant tables.)
The user table grants privileges that are
assigned to you on a global basis and that apply no matter what
the default database is. For example, if the
user table grants you the
DELETE privilege, you can delete
rows from any table in any database on the server host! It is wise
to grant privileges in the user table only to
people who need them, such as database administrators. For other
users, you should leave all privileges in the
user table set to 'N' and
grant privileges at more specific levels only. You can grant
privileges for particular databases, tables, or columns.
The db and host tables grant
database-specific privileges. Values in the scope columns of these
tables can take the following forms:
A blank User value in the
db table matches the anonymous user. A
nonblank value matches literally; there are no wildcards in
user names.
The wildcard characters “%”
and “_” can be used in the
Host and Db columns of
either table. These have the same meaning as for
pattern-matching operations performed with the
LIKE operator. If you want to use
either character literally when granting privileges, you must
escape it with a backslash. For example, to include
“_” character as part of a
database name, specify it as
“\_” in the
GRANT statement.
A '%' Host value in the
db table means “any host.” A
blank Host value in the
db table means “consult the
host table for further information”
(a process that is described later in this section).
A '%' or blank Host
value in the host table means “any
host.”
A '%' or blank Db value
in either table means “any database.”
The server reads the db and
host tables into memory and sorts them at the
same time that it reads the user table. The
server sorts the db table based on the
Host, Db, and
User scope columns, and sorts the
host table based on the Host
and Db scope columns. As with the
user table, sorting puts the most-specific
values first and least-specific values last, and when the server
looks for matching entries, it uses the first match that it finds.
The tables_priv and
columns_priv tables grant table-specific and
column-specific privileges. Values in the scope columns of these
tables can take the following forms:
The wildcard characters “%”
and “_” can be used in the
Host column. These have the same meaning as
for pattern-matching operations performed with the
LIKE operator.
A '%' or blank Host
value means “any host.”
The Db, Table_name, and
Column_name columns cannot contain
wildcards or be blank.
The server sorts the tables_priv and
columns_priv tables based on the
Host, Db, and
User columns. This is similar to
db table sorting, but simpler because only the
Host column can contain wildcards.
The server uses the sorted tables to verify each request that it
receives. For requests that require administrative privileges such
as SHUTDOWN or
RELOAD, the server checks only the
user table row because that is the only table
that specifies administrative privileges. The server grants access
if the row permits the requested operation and denies access
otherwise. For example, if you want to execute mysqladmin
shutdown but your user table row does
not grant the SHUTDOWN privilege to
you, the server denies access without even checking the
db or host tables. (They
contain no Shutdown_priv column, so there is no
need to do so.)
For database-related requests
(INSERT,
UPDATE, and so on), the server
first checks the user's global privileges by looking in the
user table row. If the row permits the
requested operation, access is granted. If the global privileges
in the user table are insufficient, the server
determines the user's database-specific privileges by checking the
db and host tables:
The server looks in the db table for a
match on the Host, Db,
and User columns. The
Host and User columns
are matched to the connecting user's host name and MySQL user
name. The Db column is matched to the
database that the user wants to access. If there is no row for
the Host and User,
access is denied.
If there is a matching db table row and its
Host column is not blank, that row defines
the user's database-specific privileges.
If the matching db table row's
Host column is blank, it signifies that the
host table enumerates which hosts should be
permitted access to the database. In this case, a further
lookup is done in the host table to find a
match on the Host and Db
columns. If no host table row matches,
access is denied. If there is a match, the user's
database-specific privileges are computed as the intersection
(not the union!) of the privileges in the
db and host table
entries; that is, the privileges that are
'Y' in both entries. (This way you can
grant general privileges in the db table
row and then selectively restrict them on a host-by-host basis
using the host table entries.)
After determining the database-specific privileges granted by the
db and host table entries,
the server adds them to the global privileges granted by the
user table. If the result permits the requested
operation, access is granted. Otherwise, the server successively
checks the user's table and column privileges in the
tables_priv and columns_priv
tables, adds those to the user's privileges, and permits or denies
access based on the result.
Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this:
global privileges OR (database privileges AND host privileges) OR table privileges OR column privileges
It may not be apparent why, if the global user
row privileges are initially found to be insufficient for the
requested operation, the server adds those privileges to the
database, table, and column privileges later. The reason is that a
request might require more than one type of privilege. For
example, if you execute an
INSERT INTO ...
SELECT statement, you need both the
INSERT and the
SELECT privileges. Your privileges
might be such that the user table row grants
one privilege and the db table row grants the
other. In this case, you have the necessary privileges to perform
the request, but the server cannot tell that from either table by
itself; the privileges granted by the entries in both tables must
be combined.
The host table is not affected by the
GRANT or
REVOKE statements, so it is unused
in most MySQL installations. If you modify it directly, you can
use it for some specialized purposes, such as to maintain a list
of secure servers on the local network that are granted all
privileges.
You can also use the host table to indicate
hosts that are not secure. Suppose that you
have a machine public.your.domain that is
located in a public area that you do not consider secure. You can
enable access to all hosts on your network except that machine by
using host table entries like this:
+--------------------+----+- | Host | Db | ... +--------------------+----+- | public.your.domain | % | ... (all privileges set to 'N') | %.your.domain | % | ... (all privileges set to 'Y') +--------------------+----+-
When mysqld starts, it reads all grant table contents into memory. The in-memory tables become effective for access control at that point.
If you modify the grant tables indirectly using account-management
statements such as GRANT,
REVOKE, or SET
PASSWORD, the server notices these changes and loads the
grant tables into memory again immediately.
If you modify the grant tables directly using statements such as
INSERT,
UPDATE, or
DELETE, your changes have no effect
on privilege checking until you either restart the server or tell
it to reload the tables. If you change the grant tables directly
but forget to reload them, your changes have no
effect until you restart the server. This may leave you
wondering why your changes seem to make no difference!
To tell the server to reload the grant tables, perform a
flush-privileges operation. This can be done by issuing a
FLUSH PRIVILEGES
statement or by executing a mysqladmin
flush-privileges or mysqladmin reload
command.
A grant table reload affects privileges for each existing client connection as follows:
Table and column privilege changes take effect with the client's next request.
Database privilege changes take effect the next time the
client executes a USE
statement.
db_name
Client applications may cache the database name; thus, this effect may not be visible to them without actually changing to a different database or flushing the privileges.
Global privileges and passwords are unaffected for a connected client. These changes take effect only for subsequent connections.
If the server is started with the
--skip-grant-tables option, it does
not read the grant tables or implement any access control. Anyone
can connect and do anything, which is
insecure. To cause a server thus started to read the
tables and enable access checking, flush the privileges.
If you encounter problems when you try to connect to the MySQL server, the following items describe some courses of action you can take to correct the problem.
Make sure that the server is running. If it is not, clients cannot connect to it. For example, if an attempt to connect to the server fails with a message such as one of those following, one cause might be that the server is not running:
shell>mysqlERROR 2003: Can't connect to MySQL server on 'host_name' (111) shell>mysqlERROR 2002: Can't connect to local MySQL server through socket '/tmp/mysql.sock' (111)
It might be that the server is running, but you are trying to
connect using a TCP/IP port, named pipe, or Unix socket file
different from the one on which the server is listening. To
correct this when you invoke a client program, specify a
--port option to indicate the
proper port number, or a
--socket option to indicate
the proper named pipe or Unix socket file. To find out where
the socket file is, you can use this command:
shell> netstat -ln | grep mysql
Make sure that the server has not been configured to ignore
network connections or (if you are attempting to connect
remotely) that it has not been configured to listen only
locally on its network interfaces. If the server was started
with --skip-networking, it will
not accept TCP/IP connections at all. If the server was
started with
--bind-address=127.0.0.1, it
will listen for TCP/IP connections only locally on the
loopback interface and will not accept remote connections.
Check to make sure that there is no firewall blocking access to MySQL. Your firewall may be configured on the basis of the application being executed, or the port number used by MySQL for communication (3306 by default). Under Linux or Unix, check your IP tables (or similar) configuration to ensure that the port has not been blocked. Under Windows, applications such as ZoneAlarm or the Windows XP personal firewall may need to be configured not to block the MySQL port.
The grant tables must be properly set up so that the server
can use them for access control. For some distribution types
(such as binary distributions on Windows, or RPM distributions
on Linux), the installation process initializes the
mysql database containing the grant tables.
For distributions that do not do this, you must initialize the
grant tables manually by running the
mysql_install_db script. For details, see
Section 2.10.2, “Unix Postinstallation Procedures”.
To determine whether you need to initialize the grant tables,
look for a mysql directory under the data
directory. (The data directory normally is named
data or var and is
located under your MySQL installation directory.) Make sure
that you have a file named user.MYD in
the mysql database directory. If not,
execute the mysql_install_db script. After
running this script and starting the server, test the initial
privileges by executing this command:
shell> mysql -u root test
The server should let you connect without error.
After a fresh installation, you should connect to the server and set up your users and their access permissions:
shell> mysql -u root mysql
The server should let you connect because the MySQL
root user has no password initially. That
is also a security risk, so setting the password for the
root accounts is something you should do
while you're setting up your other MySQL accounts. For
instructions on setting the initial passwords, see
Section 2.10.3, “Securing the Initial MySQL Accounts”.
If you have updated an existing MySQL installation to a newer version, did you run the mysql_fix_privilege_tables script? If not, do so. The structure of the grant tables changes occasionally when new capabilities are added, so after an upgrade you should always make sure that your tables have the current structure. For instructions, see Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.
If a client program receives the following error message when it tries to connect, it means that the server expects passwords in a newer format than the client is capable of generating:
shell> mysql
Client does not support authentication protocol requested
by server; consider upgrading MySQL client
For information on how to deal with this, see Section 5.4.2.3, “Password Hashing in MySQL”, and Section B.5.2.4, “Client does not support authentication protocol”.
Remember that client programs use connection parameters
specified in option files or environment variables. If a
client program seems to be sending incorrect default
connection parameters when you have not specified them on the
command line, check any applicable option files and your
environment. For example, if you get Access
denied when you run a client without any options,
make sure that you have not specified an old password in any
of your option files!
You can suppress the use of option files by a client program
by invoking it with the
--no-defaults option. For
example:
shell> mysqladmin --no-defaults -u root version
The option files that clients use are listed in Section 4.2.3.3, “Using Option Files”. Environment variables are listed in Section 2.13, “Environment Variables”.
If you get the following error, it means that you are using an
incorrect root password:
shell> mysqladmin -u root -pxxxx ver
Access denied for user 'root'@'localhost' (using password: YES)
If the preceding error occurs even when you have not specified
a password, it means that you have an incorrect password
listed in some option file. Try the
--no-defaults option as
described in the previous item.
For information on changing passwords, see Section 5.6.5, “Assigning Account Passwords”.
If you have lost or forgotten the root
password, see Section B.5.4.1, “How to Reset the Root Password”.
If you change a password by using SET
PASSWORD, INSERT, or
UPDATE, you must encrypt the
password using the PASSWORD()
function. If you do not use
PASSWORD() for these
statements, the password will not work. For example, the
following statement assigns a password, but fails to encrypt
it, so the user is not able to connect afterward:
SET PASSWORD FOR 'abe'@'host_name' = 'eagle';
Instead, set the password like this:
SET PASSWORD FOR 'abe'@'host_name' = PASSWORD('eagle');
The PASSWORD() function is
unnecessary when you specify a password using the
GRANT statement or the
mysqladmin password command. Each of those
automatically uses PASSWORD()
to encrypt the password. See
Section 5.6.5, “Assigning Account Passwords”.
localhost is a synonym for your local host
name, and is also the default host to which clients try to
connect if you specify no host explicitly. However,
connections to localhost on Unix systems do
not work if you are using a MySQL version older than 3.23.27
that uses MIT-pthreads: localhost
connections are made using Unix socket files, which were not
supported by MIT-pthreads at that time.
To avoid this problem on such systems, you can use a
--host=127.0.0.1 option to
name the server host explicitly. This will make a TCP/IP
connection to the local mysqld server. You
can also use TCP/IP by specifying a
--host option that uses the
actual host name of the local host. In this case, the host
name must be specified in a user table row
on the server host, even though you are running the client
program on the same host as the server.
The Access denied error message tells you
who you are trying to log in as, the client host from which
you are trying to connect, and whether you were using a
password. Normally, you should have one row in the
user table that exactly matches the host
name and user name that were given in the error message. For
example, if you get an error message that contains
using password: NO, it means that you tried
to log in without a password.
If you get an Access denied error when
trying to connect to the database with mysql -u
, you may have a
problem with the user_nameuser table. Check this by
executing mysql -u root mysql and issuing
this SQL statement:
SELECT * FROM user;
The result should include a row with the
Host and User columns
matching your client's host name and your MySQL user name.
If the following error occurs when you try to connect from a
host other than the one on which the MySQL server is running,
it means that there is no row in the user
table with a Host value that matches the
client host:
Host ... is not allowed to connect to this MySQL server
You can fix this by setting up an account for the combination of client host name and user name that you are using when trying to connect.
If you do not know the IP address or host name of the machine
from which you are connecting, you should put a row with
'%' as the Host column
value in the user table. After trying to
connect from the client machine, use a SELECT
USER() query to see how you really did connect. Then
change the '%' in the
user table row to the actual host name that
shows up in the log. Otherwise, your system is left insecure
because it permits connections from any host for the given
user name.
(Note that if you are running a version of MySQL older than
3.23.11, the output from USER()
does not include the host name. In this case, you must restart
the server with the --log
option, then obtain the host name from the log.)
On Linux, another reason that this error might occur is that
you are using a binary MySQL version that is compiled with a
different version of the glibc library than
the one you are using. In this case, you should either upgrade
your operating system or glibc, or download
a source distribution of MySQL version and compile it
yourself. A source RPM is normally trivial to compile and
install, so this is not a big problem.
If you specify a host name when trying to connect, but get an error message where the host name is not shown or is an IP address, it means that the MySQL server got an error when trying to resolve the IP address of the client host to a name:
shell> mysqladmin -u root -pxxxx -h some_hostname ver
Access denied for user 'root'@'' (using password: YES)
If you try to connect as root and get the
following error, it means that you do not have a row in the
user table with a User
column value of 'root' and that
mysqld cannot resolve the host name for
your client:
Access denied for user ''@'unknown'
These errors indicate a DNS problem. To fix it, execute mysqladmin flush-hosts to reset the internal DNS host name cache. See Section 7.8.5, “How MySQL Uses DNS”.
Some permanent solutions are:
Determine what is wrong with your DNS server and fix it.
Specify IP addresses rather than host names in the MySQL grant tables.
Put an entry for the client machine name in
/etc/hosts on Unix or
\windows\hosts on Windows.
Start mysqld with the
--skip-name-resolve option.
Start mysqld with the
--skip-host-cache option.
On Unix, if you are running the server and the client on
the same machine, connect to localhost.
Unix connections to localhost use a
Unix socket file rather than TCP/IP.
On Windows, if you are running the server and the client
on the same machine and the server supports named pipe
connections, connect to the host name .
(period). Connections to . use a named
pipe rather than TCP/IP.
If mysql -u root test works but
mysql -h results in your_hostname -u
root testAccess
denied (where
your_hostname is the actual host
name of the local host), you may not have the correct name for
your host in the user table. A common
problem here is that the Host value in the
user table row specifies an unqualified
host name, but your system's name resolution routines return a
fully qualified domain name (or vice versa). For example, if
you have an entry with host 'pluto' in the
user table, but your DNS tells MySQL that
your host name is 'pluto.example.com', the
entry does not work. Try adding an entry to the
user table that contains the IP address of
your host as the Host column value.
(Alternatively, you could add an entry to the
user table with a Host
value that contains a wildcard; for example,
'pluto.%'. However, use of
Host values ending with
“%” is
insecure and is not
recommended!)
If mysql -u works but user_name
testmysql -u
does not, you
have not granted access to the given user for the database
named user_name
other_dbother_db.
If mysql -u
works when
executed on the server host, but user_namemysql -h
does not work
when executed on a remote client host, you have not enabled
access to the server for the given user name from the remote
host.
host_name -u
user_name
If you cannot figure out why you get Access
denied, remove from the user
table all entries that have Host values
containing wildcards (entries that contain
'%' or '_' characters).
A very common error is to insert a new entry with
Host='%' and
User=',
thinking that this enables you to specify
some_user'localhost to connect from the same machine.
The reason that this does not work is that the default
privileges include an entry with
Host='localhost' and
User=''. Because that
entry has a Host value
'localhost' that is more specific than
'%', it is used in preference to the new
entry when connecting from localhost! The
correct procedure is to insert a second entry with
Host='localhost' and
User=',
or to delete the entry with
some_user'Host='localhost' and
User=''. After deleting
the entry, remember to issue a
FLUSH
PRIVILEGES statement to reload the grant tables. See
also Section 5.5.4, “Access Control, Stage 1: Connection Verification”.
If you are able to connect to the MySQL server, but get an
Access denied message whenever you issue a
SELECT ... INTO
OUTFILE or
LOAD DATA
INFILE statement, your entry in the
user table does not have the
FILE privilege enabled.
If you change the grant tables directly (for example, by using
INSERT,
UPDATE, or
DELETE statements) and your
changes seem to be ignored, remember that you must execute a
FLUSH
PRIVILEGES statement or a mysqladmin
flush-privileges command to cause the server to
reload the privilege tables. Otherwise, your changes have no
effect until the next time the server is restarted. Remember
that after you change the root password
with an UPDATE statement, you
will not need to specify the new password until after you
flush the privileges, because the server will not know you've
changed the password yet!
If your privileges seem to have changed in the middle of a session, it may be that a MySQL administrator has changed them. Reloading the grant tables affects new client connections, but it also affects existing connections as indicated in Section 5.5.6, “When Privilege Changes Take Effect”.
If you have access problems with a Perl, PHP, Python, or ODBC
program, try to connect to the server with mysql -u
or user_name
db_namemysql
-u . If you are able
to connect using the mysql client, the
problem lies with your program, not with the access
privileges. (There is no space between user_name
-pyour_pass
db_name-p and
the password; you can also use the
--password=
syntax to specify the password. If you use the
your_pass-p or
--password option with no
password value, MySQL prompts you for the password.)
For testing purposes, start the mysqld
server with the
--skip-grant-tables option.
Then you can change the MySQL grant tables and use the
mysqlaccess script to check whether your
modifications have the desired effect. When you are satisfied
with your changes, execute mysqladmin
flush-privileges to tell the
mysqld server to reload the privileges.
This enables you to begin using the new grant table contents
without stopping and restarting the server.
If you get the following error, you may have a problem with
the db or host table:
Access to database denied
If the entry selected from the db table has
an empty value in the Host column, make
sure that there are one or more corresponding entries in the
host table specifying which hosts the
db table entry applies to. This problem
occurs infrequently because the host table
is rarely used.
If everything else fails, start the mysqld
server with a debugging option (for example,
--debug=d,general,query). This
prints host and user information about attempted connections,
as well as information about each command issued. See
Section 18.4.3, “The DBUG Package”.
If you have any other problems with the MySQL grant tables and
feel you must post the problem to the mailing list, always
provide a dump of the MySQL grant tables. You can dump the
tables with the mysqldump mysql command. To
file a bug report, see the instructions at
Section 1.8, “How to Report Bugs or Problems”. In some cases, you may need to
restart mysqld with
--skip-grant-tables to run
mysqldump.
This section describes how to set up accounts for clients of your MySQL server. It discusses the following topics:
The meaning of account names and passwords as used in MySQL and how that compares to names and passwords used by your operating system
How to set up new accounts and remove existing accounts
How to change passwords
Guidelines for using passwords securely
How to use secure connections with SSL
See also Section 12.4.1, “Account Management Statements”, which describes the syntax and use for all user-management SQL statements.
MySQL stores accounts in the user table of the
mysql database. An account is defined in terms
of a user name and the client host or hosts from which the user
can connect to the server. The account may also have a password.
For information about account representation in the
user table, see
Section 5.5.2, “Privilege System Grant Tables”.
There are several distinctions between the way user names and passwords are used by MySQL and the way they are used by your operating system:
User names, as used by MySQL for authentication purposes, have
nothing to do with user names (login names) as used by Windows
or Unix. On Unix, most MySQL clients by default try to log in
using the current Unix user name as the MySQL user name, but
that is for convenience only. The default can be overridden
easily, because client programs permit any user name to be
specified with a -u or
--user option. Because this means that anyone
can attempt to connect to the server using any user name, you
cannot make a database secure in any way unless all MySQL
accounts have passwords. Anyone who specifies a user name for
an account that has no password is able to connect
successfully to the server.
MySQL user names can be up to 16 characters long. Operating system user names, because they are completely unrelated to MySQL user names, may be of a different maximum length. For example, Unix user names typically are limited to eight characters.
The limit on MySQL user name length is hard-coded in the
MySQL servers and clients, and trying to circumvent it by
modifying the definitions of the tables in the
mysql database does not
work.
You should never alter any of the tables in the
mysql database in any manner whatsoever
except by means of the procedure prescribed that is
described in Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.
Attempting to redefine MySQL's system tables in any other
fashion results in undefined (and unsupported!) behavior.
MySQL user names can be up to 16 characters long.
Changing the maximum length is not
supported. If you try to change it, for example by
changing the length of the User column in
the mysql database tables, this will result
in unpredictable behavior. (Altering privilege tables is not
supported in any case.) Operating system user names might have
a different maximum length. For example, Unix user names
typically are limited to eight characters.
It is best to use only ASCII characters for user names and passwords.
The server uses MySQL passwords stored in the
user table to authenticate client
connections using MySQL built-in authentication. These
passwords have nothing to do with passwords for logging in to
your operating system. There is no necessary connection
between the “external” password you use to log in
to a Windows or Unix machine and the password you use to
access the MySQL server on that machine.
MySQL encrypts passwords stored in the user
table using its own algorithm. This encryption is the same as
that implemented by the
PASSWORD() SQL function but
differs from that used during the Unix login process. Unix
password encryption is the same as that implemented by the
ENCRYPT() SQL function. See the
descriptions of the PASSWORD()
and ENCRYPT() functions in
Section 11.12, “Encryption and Compression Functions”.
From version 4.1 on, MySQL employs a stronger authentication
method that has better password protection during the
connection process than in earlier versions. It is secure even
if TCP/IP packets are sniffed or the mysql
database is captured. (In earlier versions, even though
passwords are stored in encrypted form in the
user table, knowledge of the encrypted
password value could be used to connect to the MySQL server.)
Section 5.4.2.3, “Password Hashing in MySQL”, discusses password
encryption further.
When you install MySQL, the grant tables are populated with an
initial set of accounts. The names and access privileges for these
accounts are described in Section 2.10.3, “Securing the Initial MySQL Accounts”,
which also discusses how to assign passwords to them. Thereafter,
you normally set up, modify, and remove MySQL accounts using
statements such as GRANT and
REVOKE. See
Section 12.4.1, “Account Management Statements”.
When you connect to a MySQL server with a command-line client, specify the user name and password as necessary for the account that you want to use:
shell> mysql --user=monty --password=password db_name
If you prefer short options, the command looks like this:
shell> mysql -u monty -ppassword db_name
There must be no space between the
-p option and the following password value.
If you omit the password value
following the --password or
-p option on the command line, the client prompts
for one.
Specifying a password on the command line should be considered insecure. See Section 5.4.2.2, “End-User Guidelines for Password Security”. You can use an option file to avoid giving the password on the command line.
For additional information about specifying user names, passwords, and other connection parameters, see Section 4.2.2, “Connecting to the MySQL Server”.
You can create MySQL accounts in two ways:
The preferred method is to use
GRANT statements, because they are
more concise and less error-prone than manipulating the grant
tables directly. GRANT is described
in Section 12.4.1.2, “GRANT Syntax”.
Another option for creating accounts is to use one of several
available third-party programs that offer capabilities for MySQL
account administration. phpMyAdmin is one such
program.
The following examples show how to use the
mysql client program to set up new accounts.
These examples assume that privileges have been set up according
to the defaults described in Section 2.10.3, “Securing the Initial MySQL Accounts”.
This means that to make changes, you must connect to the MySQL
server as the MySQL root user, and the
root account must have the
INSERT privilege for the
mysql database and the
RELOAD administrative privilege.
First, use the mysql program to connect to the
server as the MySQL root user:
shell> mysql --user=root mysql
If you have assigned a password to the root
account, you also need to supply a --password or
-p option, both for this mysql
command and for those later in this section.
After connecting to the server as root, you can
add new accounts. The following statements use
GRANT to set up four new accounts:
mysql>GRANT ALL PRIVILEGES ON *.* TO 'monty'@'localhost'->IDENTIFIED BY 'some_pass' WITH GRANT OPTION;mysql>GRANT ALL PRIVILEGES ON *.* TO 'monty'@'%'->IDENTIFIED BY 'some_pass' WITH GRANT OPTION;mysql>GRANT RELOAD,PROCESS ON *.* TO 'admin'@'localhost';mysql>GRANT USAGE ON *.* TO 'dummy'@'localhost';
The accounts created by these statements have the following properties:
Two of the accounts have a user name of
monty and a password of
some_pass. Both accounts are superuser
accounts with full privileges to do anything. The
'monty'@'localhost' account can be used
only when connecting from the local host. The
'monty'@'%' account uses the
'%' wildcard for the host part, so it can
be used to connect from any host.
It is necessary to have both accounts for
monty to be able to connect from anywhere
as monty. Without the
localhost account, the anonymous-user
account for localhost that is created by
mysql_install_db would take precedence when
monty connects from the local host. As a
result, monty would be treated as an
anonymous user. The reason for this is that the anonymous-user
account has a more specific Host column
value than the 'monty'@'%' account and thus
comes earlier in the user table sort order.
(user table sorting is discussed in
Section 5.5.4, “Access Control, Stage 1: Connection Verification”.)
The 'admin'@'localhost' account has no
password. This account can be used only by
admin to connect from the local host. It is
granted the RELOAD and
PROCESS administrative
privileges. These privileges enable the
admin user to execute the
mysqladmin reload, mysqladmin
refresh, and mysqladmin
flush-xxx commands, as
well as mysqladmin processlist . No
privileges are granted for accessing any databases. You could
add such privileges later by issuing other
GRANT statements.
The 'dummy'@'localhost' account has no
password. This account can be used only to connect from the
local host. No privileges are granted. The
USAGE privilege in the
GRANT statement enables you to
create an account without giving it any privileges. It has the
effect of setting all the global privileges to
'N'. It is assumed that you will grant
specific privileges to the account later.
To check the privileges for an account, use
SHOW GRANTS:
mysql> SHOW GRANTS FOR 'admin'@'localhost';
+-----------------------------------------------------+
| Grants for admin@localhost |
+-----------------------------------------------------+
| GRANT RELOAD, PROCESS ON *.* TO 'admin'@'localhost' |
+-----------------------------------------------------+
As an alternative to GRANT, you can
create the same accounts directly by issuing
INSERT statements and then telling
the server to reload the grant tables:
shell>mysql --user=root mysqlmysql>INSERT INTO user->VALUES('localhost','monty',PASSWORD('some_pass'),->'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');mysql>INSERT INTO user->VALUES('%','monty',PASSWORD('some_pass'),->'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y');mysql>INSERT INTO user SET Host='localhost',User='admin',->Reload_priv='Y', Process_priv='Y';mysql>INSERT INTO user (Host,User,Password)->VALUES('localhost','dummy','');mysql>FLUSH PRIVILEGES;
When you create accounts with
INSERT, it is necessary to use
FLUSH PRIVILEGES
to tell the server to reload the grant tables. Otherwise, the
changes go unnoticed until you restart the server. With
GRANT,
FLUSH PRIVILEGES
is unnecessary.
The reason for using the PASSWORD()
function with INSERT is to encrypt
the password. The GRANT statement
encrypts the password for you, so
PASSWORD() is unnecessary.
The 'Y' values enable privileges for the
accounts. Depending on your MySQL version, you may have to use a
different number of 'Y' values in the first two
INSERT statements. (Versions prior
to 3.22.11 have fewer privilege columns, and versions from 4.0.2
on have more.) The INSERT statement
for the admin account employs the more readable
extended INSERT syntax using
SET that is available starting with MySQL
3.22.11 is used.
In the INSERT statement for the
dummy account, only the
Host, User, and
Password columns in the user
table row are assigned values. None of the privilege columns are
set explicitly, so MySQL assigns them all the default value of
'N'. This is equivalent to what
GRANT USAGE does.
To set up a superuser account, it is necessary only to insert a
user table row with all privilege columns set
to 'Y'. The user table
privileges are global, so no entries in any of the other grant
tables are needed.
The next examples create three accounts and give them access to
specific databases. Each of them has a user name of
custom and password of
obscure.
To create the accounts with GRANT,
use the following statements:
shell>mysql --user=root mysqlmysql>GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP->ON bankaccount.*->TO 'custom'@'localhost'->IDENTIFIED BY 'obscure';mysql>GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP->ON expenses.*->TO 'custom'@'host47.example.com'->IDENTIFIED BY 'obscure';mysql>GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP->ON customer.*->TO 'custom'@'server.domain'->IDENTIFIED BY 'obscure';
The three accounts can be used as follows:
The first account can access the
bankaccount database, but only from the
local host.
The second account can access the expenses
database, but only from the host
host47.example.com.
The third account can access the customer
database, but only from the host
server.domain.
To set up the custom accounts without
GRANT, use
INSERT statements as follows to
modify the grant tables directly:
shell>mysql --user=root mysqlmysql>INSERT INTO user (Host,User,Password)->VALUES('localhost','custom',PASSWORD('obscure'));mysql>INSERT INTO user (Host,User,Password)->VALUES('host47.example.com','custom',PASSWORD('obscure'));mysql>INSERT INTO user (Host,User,Password)->VALUES('server.domain','custom',PASSWORD('obscure'));mysql>INSERT INTO db->(Host,Db,User,Select_priv,Insert_priv,->Update_priv,Delete_priv,Create_priv,Drop_priv)->VALUES('localhost','bankaccount','custom',->'Y','Y','Y','Y','Y','Y');mysql>INSERT INTO db->(Host,Db,User,Select_priv,Insert_priv,->Update_priv,Delete_priv,Create_priv,Drop_priv)->VALUES('host47.example.com','expenses','custom',->'Y','Y','Y','Y','Y','Y');mysql>INSERT INTO db->(Host,Db,User,Select_priv,Insert_priv,->Update_priv,Delete_priv,Create_priv,Drop_priv)->VALUES('server.domain','customer','custom',->'Y','Y','Y','Y','Y','Y');mysql>FLUSH PRIVILEGES;
The first three INSERT statements
add user table entries that permit the user
custom to connect from the various hosts with
the given password, but grant no global privileges (all privileges
are set to the default value of 'N'). The next
three INSERT statements add
db table entries that grant privileges to
custom for the bankaccount,
expenses, and customer
databases, but only when accessed from the proper hosts. As usual
when you modify the grant tables directly, you must tell the
server to reload them with
FLUSH PRIVILEGES
so that the privilege changes take effect.
To create a user who has access from all machines in a given
domain (for example, mydomain.com), you can use
the “%” wildcard character in the
host part of the account name:
mysql>GRANT ...->ON *.*->TO 'myname'@'%.mydomain.com'->IDENTIFIED BY 'mypass';
To do the same thing by modifying the grant tables directly, do this:
mysql>INSERT INTO user (Host,User,Password,...)->VALUES('%.mydomain.com','myname',PASSWORD('mypass'),...);mysql>FLUSH PRIVILEGES;
To remove an account, use the DROP
USER statement, which was added in MySQL 4.1.1. For
older versions of MySQL, use DELETE
instead. The account removal procedure is described in
Section 12.4.1.1, “DROP USER Syntax”.
Before MySQL 4.0.2, the only means of limiting use of MySQL server
resources is to set the global
max_user_connections system
variable to a nonzero value. This limits the number of
simultaneous connections that can be made by any given account,
but places no limits on what a client can do once connected. In
addition, this method is strictly global, and does not enable
management of individual accounts. Both types of control are of
interest to many MySQL administrators, particularly those working
for Internet Service Providers.
Starting from MySQL 4.0.2, you can limit access to the following server resources for individual accounts:
The number of queries that an account can issue per hour
The number of updates that an account can issue per hour
The number of times an account can connect to the server per hour
Any statement that a client can issue counts against the query limit (unless its results are served from the query cache). Only statements that modify databases or tables count against the update limit.
An “account” in this context is assessed against the
actual host from which a user connects. Suppose that there is a
row in the user table that has
User and Host values of
usera and %.example.com, to
permit usera to connect from any host in the
example.com domain. If usera
connects simultaneously from host1.example.com
and host2.example.com, the server applies the
account resource limits separately to each connection. If
usera connects again from
host1.example.com, the server applies the
limits for that connection together with the existing connection
from that host.
The server limits account resources based on the resource-related
columns of the user table in the
mysql database:
max_questions, max_updates,
max_connections, and
max_user_connections. If your
user table does not have these columns, it must
be upgraded; see Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.
To set resource limits for an account, use the
GRANT statement (see
Section 12.4.1.2, “GRANT Syntax”). Provide a WITH clause
that names each resource to be limited. For example, to create a
new account that can access the customer
database, but only in a limited fashion, issue this statement:
mysql>GRANT ALL ON customer.* TO 'francis'@'localhost'->IDENTIFIED BY 'frank'->WITH MAX_QUERIES_PER_HOUR 20->MAX_UPDATES_PER_HOUR 10->MAX_CONNECTIONS_PER_HOUR 5;
The limit types need not all be named in the
WITH clause, but those named can be present in
any order. The value for each per-hour limit should be an integer
representing a count per hour. If the
GRANT statement has no
WITH clause, the limits are each set to the
default value of zero (that is, no limit).
To modify existing limits for an account, use a
GRANT USAGE
statement at the global level (ON *.*). The
following statement changes the query limit for
francis to 100:
mysql>GRANT USAGE ON *.* TO 'francis'@'localhost'->WITH MAX_QUERIES_PER_HOUR 100;
The statement modifies only the limit value specified and leaves the account otherwise unchanged.
To remove a limit, set its value to zero. For example, to remove
the limit on how many times per hour francis
can connect, use this statement:
mysql>GRANT USAGE ON *.* TO 'francis'@'localhost'->WITH MAX_CONNECTIONS_PER_HOUR 0;
The server stores resource limits for an account in the
user table row corresponding to the account.
The max_questions,
max_updates, and
max_connections columns store the per-hour
limits. (See Section 5.5.2, “Privilege System Grant Tables”.)
Resource-use counting takes place when any account has a nonzero limit placed on its use of any of the resources.
As the server runs, it counts the number of times each account uses resources. If an account reaches its limit on number of connections within the last hour, further connections for the account are rejected until that hour is up. Similarly, if the account reaches its limit on the number of queries or updates, further queries or updates are rejected until the hour is up. In all such cases, an appropriate error message is issued.
The current per-hour resource-use counts can be reset globally for all accounts, or individually for a given account:
To reset the current counts to zero for all accounts, issue a
FLUSH
USER_RESOURCES statement. The counts also can be
reset by reloading the grant tables (for example, with a
FLUSH
PRIVILEGES statement or a mysqladmin
reload command).
The counts for an individual account can be set to zero by
re-granting it any of its limits. To do this, use
GRANT USAGE
as described earlier and specify a limit value equal to the
value that the account currently has.
All counts begin at zero when the server starts; counts are not carried over through a restart.
Required credentials for clients that connect to the MySQL server can include a password. This section describes how to assign passwords for MySQL accounts.
To assign a password when you create a new account with
GRANT, include an
IDENTIFIED BY clause:
mysql>GRANT ALL ON db.* TO 'jeffrey'@'localhost'->IDENTIFIED BY 'mypass';
To assign or change a password for an existing account, one way is
to issue a SET PASSWORD statement:
mysql>SET PASSWORD FOR->'jeffrey'@'localhost' = PASSWORD('mypass');
MySQL stores passwords in the user table in the
mysql database. Only users such as
root that have update access to the
mysql database can change the password for
other users. If you are not connected as an anonymous user, you
can change your own password by omitting the
FOR clause:
mysql> SET PASSWORD = PASSWORD('mypass');
You can also use a GRANT
USAGE statement at the global level (ON
*.*) to assign a password to an account without
affecting the account's current privileges:
mysql>GRANT USAGE ON *.* TO 'jeffrey'@'localhost'->IDENTIFIED BY 'mypass';
To assign a password from the command line, use the mysqladmin command:
shell> mysqladmin -u user_name -h host_name password "newpwd"
The account for which this command sets the password is the one
with a user table row that matches
user_name in the
User column and the client host from
which you connect in the Host
column.
It is preferable to assign passwords using one of the preceding
methods, but it is also possible to modify the
user table directly. In this case, you must
also use FLUSH
PRIVILEGES to cause the server to reread the grant
tables. Otherwise, the change remains unnoticed by the server
until you restart it.
To establish a password for a new account, provide a value for
the Password column:
mysql>INSERT INTO mysql.user (Host,User,Password)->VALUES('localhost','jeffrey',PASSWORD('mypass'));mysql>FLUSH PRIVILEGES;
To change the password for an existing account, use
UPDATE to set the
Password column value:
mysql>UPDATE mysql.user SET Password = PASSWORD('bagel')->WHERE Host = 'localhost' AND User = 'francis';mysql>FLUSH PRIVILEGES;
During authentication when a client connects to the server, MySQL
treats the password in the user table as an
encrypted hash value (the value that
PASSWORD() would return for the
password). When assigning a password to an account, it is
important to store an encrypted value, not the plaintext password.
Use the following guidelines:
When you assign a password using
GRANT with an
IDENTIFIED BY clause or with the
mysqladmin password command, they encrypt
the password for you. Specify the literal plaintext password:
mysql>GRANT USAGE ON *.* TO 'jeffrey'@'localhost'->IDENTIFIED BY 'mypass';
For GRANT, you can avoid
sending the plaintext password if you know the hashed value
that PASSWORD() would return
for the password. Specify the hashed value preceded by the
keyword PASSWORD:
mysql>GRANT USAGE ON *.* TO 'jeffrey'@'localhost'->IDENTIFIED BY PASSWORD '*90E462C37378CED12064BB3388827D2BA3A9B689';
When you assign an account a nonempty password using
SET PASSWORD,
INSERT, or
UPDATE, you must use the
PASSWORD() function to encrypt
the password, otherwise the password is stored as plaintext.
Suppose that you assign a password like this:
mysql>SET PASSWORD FOR->'jeffrey'@'localhost' = 'mypass';
The result is that the literal value
'mypass' is stored as the password in the
user table, not the encrypted value. When
jeffrey attempts to connect to the server
using this password, the value is encrypted and compared to
the value stored in the user table.
However, the stored value is the literal string
'mypass', so the comparison fails and the
server rejects the connection with an Access
denied error.
PASSWORD() encryption differs
from Unix password encryption. See Section 5.6.1, “User Names and Passwords”.
Beginning with version 4.0.0, MySQL has support for secure
(encrypted) connections between MySQL clients and the server using
the Secure Sockets Layer (SSL) protocol. This section discusses
how to use SSL connections. For information on how to require
users to use SSL connections, see the discussion of the
REQUIRE clause of the
GRANT statement in
Section 12.4.1.2, “GRANT Syntax”.
The standard configuration of MySQL is intended to be as fast as possible, so encrypted connections are not used by default. Doing so would make the client/server protocol much slower. Encrypting data is a CPU-intensive operation that requires the computer to do additional work and can delay other MySQL tasks. For applications that require the security provided by encrypted connections, the extra computation is warranted.
MySQL enables encryption on a per-connection basis. You can choose a normal unencrypted connection or a secure encrypted SSL connection according the requirements of individual applications.
Secure connections are based on the OpenSSL API and are available through the MySQL C API. Replication uses the C API, so secure connections can be used between master and slave servers.
Another way to connect securely is from within an SSH connection to the MySQL server host. For an example, see Section 5.6.7, “Connecting to MySQL Remotely from Windows with SSH”.
To understand how MySQL uses SSL, it is necessary to explain some basic SSL and X509 concepts. People who are familiar with these can skip this part of the discussion.
By default, MySQL uses unencrypted connections between the
client and the server. This means that someone with access to
the network could watch all your traffic and look at the data
being sent or received. They could even change the data while it
is in transit between client and server. To improve security a
little, you can compress client/server traffic by using the
--compress option when invoking client
programs. However, this does not foil a determined attacker.
When you need to move information over a network in a secure fashion, an unencrypted connection is unacceptable. Encryption is the way to make any kind of data unreadable. In fact, today's practice requires many additional security elements from encryption algorithms. They should resist many kind of known attacks such as changing the order of encrypted messages or replaying data twice.
SSL is a protocol that uses different encryption algorithms to ensure that data received over a public network can be trusted. It has mechanisms to detect any data change, loss, or replay. SSL also incorporates algorithms that provide identity verification using the X509 standard.
X509 makes it possible to identify someone on the Internet. It is most commonly used in e-commerce applications. In basic terms, there should be some company called a “Certificate Authority” (or CA) that assigns electronic certificates to anyone who needs them. Certificates rely on asymmetric encryption algorithms that have two encryption keys (a public key and a secret key). A certificate owner can show the certificate to another party as proof of identity. A certificate consists of its owner's public key. Any data encrypted with this public key can be decrypted only using the corresponding secret key, which is held by the owner of the certificate.
If you need more information about SSL, X509, or encryption, use your favorite Internet search engine to search for the keywords in which you are interested.
To use SSL connections between the MySQL server and client programs, your system must support OpenSSL and your version of MySQL must be 4.0.0 or newer and built with SSL support.
To get secure connections to work with MySQL and SSL, you must do the following:
Install the OpenSSL library if it has not already been installed. We have tested MySQL with OpenSSL 0.9.6. To obtain OpenSSL, visit http://www.openssl.org.
Building MySQL using OpenSSL requires a shared OpenSSL library, otherwise linker errors occur.
If you are not using a binary (precompiled) version of MySQL
that has been built with SSL support, configure a MySQL
source distribution to use SSL. When you configure MySQL,
invoke the configure script with the
--with-vio and
--with-openssl options:
shell> ./configure --with-vio --with-openssl
Make sure that the user in the
mysql database includes the SSL-related
columns (beginning with ssl_ and
x509_). If your user
table does not have these columns, it must be upgraded; see
Section 4.4.5, “mysql_fix_privilege_tables — Upgrade MySQL System Tables”.
To check whether a server binary is compiled with SSL
support, invoke it with the
--ssl option. An error will
occur if the server does not support SSL:
shell> mysqld --ssl --help
060525 14:18:52 [ERROR] mysqld: unknown option '--ssl'
To check whether a running mysqld server
supports SSL, examine the value of the
have_openssl system
variable:
mysql> SHOW VARIABLES LIKE 'have_openssl';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| have_openssl | YES |
+---------------+-------+
If the value is YES, the server supports
OpenSSL connections.
To enable SSL connections, the proper SSL-related options must be used (see Section 5.6.6.3, “SSL Command Options”).
To start the MySQL server so that it permits clients to connect using SSL, use the options that identify the key and certificate files the server needs when establishing a secure connection:
shell>mysqld --ssl-ca=ca-cert.pem\--ssl-cert=server-cert.pem\--ssl-key=server-key.pem
--ssl-ca identifies the
Certificate Authority (CA) certificate.
--ssl-cert identifies the
server public key. This can be sent to the client and
authenticated against the CA certificate that it has.
--ssl-key identifies the
server private key.
To establish a secure connection to a MySQL server with SSL
support, the options that a client must specify depend on the
SSL requirements of the user account that the client uses. (See
the discussion of the REQUIRE clause in
Section 12.4.1.2, “GRANT Syntax”.)
If the account has no special SSL requirements or was created
using a GRANT statement that
includes the REQUIRE SSL option, a client can
connect securely by using just the
--ssl-ca option:
shell> mysql --ssl-ca=ca-cert.pem
To require that a client certificate also be specified, create
the account using the REQUIRE X509 option.
Then the client must also specify the proper client key and
certificate files or the server will reject the connection:
shell>mysql --ssl-ca=ca-cert.pem\--ssl-cert=client-cert.pem\--ssl-key=client-key.pem
In other words, the options are similar to those used for the server. Note that the Certificate Authority certificate has to be the same.
A client can determine whether the current connection with the
server uses SSL by checking the value of the
Ssl_cipher status variable.
The value of Ssl_cipher is
nonempty if SSL is used, and empty otherwise. For example:
mysql> SHOW STATUS LIKE 'Ssl_cipher';
+---------------+--------------------+
| Variable_name | Value |
+---------------+--------------------+
| Ssl_cipher | DHE-RSA-AES256-SHA |
+---------------+--------------------+
For the mysql client, you can use the
STATUS or \s command and
check the SSL line:
mysql> \s
...
SSL: Not in use
...
Or:
mysql> \s
...
SSL: Cipher in use is DHE-RSA-AES256-SHA
...
To establish a secure connection from within an application
program, use the mysql_ssl_set()
C API function to set the appropriate certificate options before
calling mysql_real_connect().
See Section 17.6.6.65, “mysql_ssl_set()”.
The following list describes options that are used for
specifying the use of SSL, certificate files, and key files.
These options are available beginning with MySQL 4.0. They can
be given on the command line or in an option file. These options
are not available unless MySQL has been built with SSL support.
See Section 5.6.6.2, “Using SSL Connections”. (There are also
--master-ssl* options that can be used for
setting up a secure connection from a slave replication server
to a master server; see Section 14.8, “Replication and Binary Logging Options and Variables”.)
Table 5.11 SSL Option/Variable Summary
| Name | Cmd-Line | Option File | System Var | Status Var | Var Scope | Dynamic |
|---|---|---|---|---|---|---|
| have_openssl | Yes | Global | No | |||
| skip-ssl | Yes | Yes | ||||
| ssl | Yes | Yes | ||||
| ssl-ca | Yes | Yes | Global | No | ||
| - Variable: ssl_ca | Yes | Global | No | |||
| ssl-capath | Yes | Yes | Global | No | ||
| - Variable: ssl_capath | Yes | Global | No | |||
| ssl-cert | Yes | Yes | Global | No | ||
| - Variable: ssl_cert | Yes | Global | No | |||
| ssl-cipher | Yes | Yes | Global | No | ||
| - Variable: ssl_cipher | Yes | Global | No | |||
| ssl-key | Yes | Yes | Global | No | ||
| - Variable: ssl_key | Yes | Global | No |
For the server, this option specifies that the server
permits SSL connections. For a client program, it permits
the client to connect to the server using SSL. This option
is not sufficient in itself to cause an SSL connection to be
used. You must also specify the
--ssl-ca option, and
possibly the --ssl-cert and
--ssl-key options.
This option is more often used in its opposite form to
override any other SSL options and indicate that SSL should
not be used. To do this, specify the
option as
--skip-ssl
or --ssl=0.
Note that use of --ssl does
not require an SSL connection. For
example, if the server or client is compiled without SSL
support, a normal unencrypted connection is used.
The secure way to require use of an SSL connection is to
create an account on the server that includes a
REQUIRE SSL clause in the
GRANT statement. Then use
that account to connect to the server, where both the server
and the client have SSL support enabled.
The REQUIRE clause permits other
SSL-related restrictions as well. The description of
REQUIRE in Section 12.4.1.2, “GRANT Syntax”,
provides additional detail about which SSL command options
may or must be specified by clients that connect using
accounts that are created using the various
REQUIRE options.
The path to a file that contains a list of trusted SSL CAs.
The path to a directory that contains trusted SSL CA certificates in PEM format.
The name of the SSL certificate file to use for establishing a secure connection.
A list of permissible ciphers to use for SSL encryption. For
greatest portability, cipher_list
should be a list of one or more cipher names, separated by
colons. Examples:
--ssl-cipher=AES128-SHA --ssl-cipher=DHE-RSA-AES256-SHA:AES128-SHA
This format is understood both by OpenSSL and yaSSL. OpenSSL supports a more flexible syntax for specifying ciphers, as described in the OpenSSL documentation at http://www.openssl.org/docs/apps/ciphers.html. However, this extended syntax will fail if used with a MySQL installation compiled against yaSSL (which may be the case for MySQL 5.0 and up).
If no cipher in the list is supported, SSL connections will not work.
The name of the SSL key file to use for establishing a secure connection.
This section demonstrates how to set up SSL certificate and key files for use by MySQL servers and clients. The first example shows a simplified procedure such as you might use from the command line. The second shows a script that contains more detail. The first two examples are intended for use on Unix and both use the openssl command that is part of OpenSSL. The third example describes how to set up SSL files on Windows.
Following the third example, instructions are given for using the files to test SSL connections. You can also use the files as described in Section 5.6.6.2, “Using SSL Connections”.
Example 1: Creating SSL files from the command line on Unix
The following example shows a set of commands to create MySQL server and client certificate and key files. You will need to respond to several prompts by the openssl commands. For testing, you can press Enter to all prompts. For production use, you should provide nonempty responses.
# Create clean environment shell>rm -rf newcertsshell>mkdir newcerts && cd newcerts# Create CA certificate shell>openssl genrsa 2048 > ca-key.pemshell>openssl req -new -x509 -nodes -days 1000 \-key ca-key.pem > ca-cert.pem# Create server certificate shell>openssl req -newkey rsa:2048 -days 1000 \-nodes -keyout server-key.pem > server-req.pemshell>openssl x509 -req -in server-req.pem -days 1000 \-CA ca-cert.pem -CAkey ca-key.pem -set_serial 01 > server-cert.pem# Create client certificate shell>openssl req -newkey rsa:2048 -days 1000 \-nodes -keyout client-key.pem > client-req.pemshell>openssl x509 -req -in client-req.pem -days 1000 \-CA ca-cert.pem -CAkey ca-key.pem -set_serial 01 > client-cert.pem
Example 2: Creating SSL files using a script on Unix
Here is an example script that shows how to set up SSL certificates for MySQL:
DIR=`pwd`/openssl
PRIV=$DIR/private
mkdir $DIR $PRIV $DIR/newcerts
cp /usr/share/ssl/openssl.cnf $DIR
replace ./demoCA $DIR -- $DIR/openssl.cnf
# Create necessary files: $database, $serial and $new_certs_dir
# directory (optional)
touch $DIR/index.txt
echo "01" > $DIR/serial
#
# Generation of Certificate Authority(CA)
#
openssl req -new -x509 -keyout $PRIV/cakey.pem -out $DIR/ca-cert.pem \
-days 3600 -config $DIR/openssl.cnf
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ................++++++
# .........++++++
# writing new private key to '/home/monty/openssl/private/cakey.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL admin
# Email Address []:
#
# Create server request and key
#
openssl req -new -keyout $DIR/server-key.pem -out \
$DIR/server-req.pem -days 3600 -config $DIR/openssl.cnf
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# ..++++++
# ..........++++++
# writing new private key to '/home/monty/openssl/server-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL server
# Email Address []:
#
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:
#
# Remove the passphrase from the key
#
openssl rsa -in $DIR/server-key.pem -out $DIR/server-key.pem
#
# Sign server cert
#
openssl ca -policy policy_anything -out $DIR/server-cert.pem \
-config $DIR/openssl.cnf -infiles $DIR/server-req.pem
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName :PRINTABLE:'FI'
# organizationName :PRINTABLE:'MySQL AB'
# commonName :PRINTABLE:'MySQL admin'
# Certificate is to be certified until Sep 13 14:22:46 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
#
#
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated
#
# Create client request and key
#
openssl req -new -keyout $DIR/client-key.pem -out \
$DIR/client-req.pem -days 3600 -config $DIR/openssl.cnf
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Generating a 1024 bit RSA private key
# .....................................++++++
# .............................................++++++
# writing new private key to '/home/monty/openssl/client-key.pem'
# Enter PEM pass phrase:
# Verifying password - Enter PEM pass phrase:
# -----
# You are about to be asked to enter information that will be
# incorporated into your certificate request.
# What you are about to enter is what is called a Distinguished Name
# or a DN.
# There are quite a few fields but you can leave some blank
# For some fields there will be a default value,
# If you enter '.', the field will be left blank.
# -----
# Country Name (2 letter code) [AU]:FI
# State or Province Name (full name) [Some-State]:.
# Locality Name (eg, city) []:
# Organization Name (eg, company) [Internet Widgits Pty Ltd]:MySQL AB
# Organizational Unit Name (eg, section) []:
# Common Name (eg, YOUR name) []:MySQL user
# Email Address []:
#
# Please enter the following 'extra' attributes
# to be sent with your certificate request
# A challenge password []:
# An optional company name []:
#
# Remove the passphrase from the key
#
openssl rsa -in $DIR/client-key.pem -out $DIR/client-key.pem
#
# Sign client cert
#
openssl ca -policy policy_anything -out $DIR/client-cert.pem \
-config $DIR/openssl.cnf -infiles $DIR/client-req.pem
# Sample output:
# Using configuration from /home/monty/openssl/openssl.cnf
# Enter PEM pass phrase:
# Check that the request matches the signature
# Signature ok
# The Subjects Distinguished Name is as follows
# countryName :PRINTABLE:'FI'
# organizationName :PRINTABLE:'MySQL AB'
# commonName :PRINTABLE:'MySQL user'
# Certificate is to be certified until Sep 13 16:45:17 2003 GMT
# (365 days)
# Sign the certificate? [y/n]:y
#
#
# 1 out of 1 certificate requests certified, commit? [y/n]y
# Write out database with 1 new entries
# Data Base Updated
#
# Create a my.cnf file that you can use to test the certificates
#
cnf=""
cnf="$cnf [client]"
cnf="$cnf ssl-ca=$DIR/ca-cert.pem"
cnf="$cnf ssl-cert=$DIR/client-cert.pem"
cnf="$cnf ssl-key=$DIR/client-key.pem"
cnf="$cnf [mysqld]"
cnf="$cnf ssl-ca=$DIR/ca-cert.pem"
cnf="$cnf ssl-cert=$DIR/server-cert.pem"
cnf="$cnf ssl-key=$DIR/server-key.pem"
echo $cnf | replace " " '
' > $DIR/my.cnf
Example 3: Creating SSL files on Windows
Download OpenSSL for Windows. An overview of available packages can be seen here: http://www.slproweb.com/products/Win32OpenSSL.html
Choose of the following packages, depending on your architecture (32-bit or 64-bit):
Win32 OpenSSL v0.9.8l Light, available at: http://www.slproweb.com/download/Win32OpenSSL_Light-0_9_8l.exe
Win64 OpenSSL v0.9.8l Light, available at: http://www.slproweb.com/download/Win64OpenSSL_Light-0_9_8l.exe
if a message occurs during setup indicating
'...critical component is missing: Microsoft Visual C++
2008 Redistributables', cancel the setup and download
one of the following packages as well, again depending on your
architecture (32-bit or 64-bit):
Visual C++ 2008 Redistributables (x86), available at: http://www.microsoft.com/downloads/details.aspx?familyid=9B2DA534-3E03-4391-8A4D-074B9F2BC1BF“isplaylang=en
Visual C++ 2008 Redistributables (x64), available at: http://www.microsoft.com/downloads/details.aspx?familyid=bd2a6171-e2d6-4230-b809-9a8d7548c1b6“isplaylang=en
After installing the additional package, restart the OpenSSL setup.
During installation, leave the default
C:\OpenSSL as the install path, and also
leave the default option 'Copy OpenSSL DLL files to the
Windows system directory' selected.
When the installation has finished, add
C:\OpenSSL\bin to the Windows System Path
variable of your server:
On the Windows desktop, right-click the My Computer icon, and select .
Next select the tab from the menu that appears, and click the button.
Under System Variables, select , and then click the button. The dialogue should appear.
Add ';C:\OpenSSL\bin' to the end (notice
the semicolon).
Press OK 3 times.
Check that OpenSSL was correctly integrated into the Path variable by opening a new command console (Start>Run>cmd.exe) and verifying that OpenSSL is available:
Microsoft Windows [Version ...] Copyright (c) 2006 Microsoft Corporation. All rights reserved. C:\Windows\system32>cd \C:\>opensslOpenSSL>exit<<< If you see the OpenSSL prompt, installation was successful. C:\>
Depending on your version of Windows, the preceding instructions might be slightly different.
After OpenSSL has been installed, use the instructions from Example 1 (shown earlier in this section), with the following changes:
Change the follow Unix commands:
# Create clean environment shell>rm -rf newcertsshell>mkdir newcerts && cd newcerts
On Windows, use these commands instead:
# Create clean environment shell>md c:\newcertsshell>cd c:\newcerts
When a '\' character is shown at the end
of a command line, this '\' character
must be removed and the command lines entered all on a
single line.
For references to my.cnf option files,
substitute my.ini instead.
Testing SSL connections
To test SSL connections, start the server as follows, where
$DIR is the path name to the directory where
the sample my.cnf option file is located:
shell> mysqld --defaults-file=$DIR/my.cnf &
Then invoke a client program using the same option file:
shell> mysql --defaults-file=$DIR/my.cnf
If you have a MySQL source distribution, you can also test your
setup by modifying the preceding my.cnf
file to refer to the demonstration certificate and key files in
the SSL directory of the distribution.
This section describes how to get a secure connection to a remote
MySQL server with SSH. The information was provided by David
Carlson <dcarlson@mplcomm.com>.
Install an SSH client on your Windows machine. As a user, the
best nonfree one I have found is from
SecureCRT from
http://www.vandyke.com/. Another option is
f-secure from
http://www.f-secure.com/. You can also find
some free ones on Google at
http://directory.google.com/Top/Computers/Internet/Protocols/SSH/Clients/Windows/.
Start your Windows SSH client. Set Host_Name =
.
Set
yourmysqlserver_URL_or_IPuserid=
to log in to your server. This your_useriduserid value
might not be the same as the user name of your MySQL account.
Set up port forwarding. Either do a remote forward (Set
local_port: 3306, remote_host:
,
yourmysqlservername_or_ipremote_port: 3306 ) or a local forward (Set
port: 3306, host:
localhost, remote port: 3306).
Save everything, otherwise you will have to redo it the next time.
Log in to your server with the SSH session you just created.
On your Windows machine, start some ODBC application (such as Access).
Create a new file in Windows and link to MySQL using the ODBC
driver the same way you normally do, except type in
localhost for the MySQL host server, not
yourmysqlservername.
At this point, you should have an ODBC connection to MySQL, encrypted using SSH.
Applications can use the following guidelines to perform auditing that ties database activity to MySQL accounts.
MySQL accounts correspond to rows in the
mysql.user table. When a client connects
successfully, the server authenticates the client to a particular
row in this table. The User and
Host column values in this row uniquely
identify the account and correspond to the
'
format in which account names are written in SQL statements.
user_name'@'host_name'
The account used to authenticate a client determines which
privileges the client has. Normally, the
CURRENT_USER() function can be
invoked to determine which account this is for the client user.
Its value is constructed from the User and
Host columns of the user
table row for the account.
To determine the invoking user, you can also call the
USER() function, which returns a
value indicating the actual user name provided by the client and
the host from which the client connected. However, this value does
not necessarily correspond directly to an account in the
user table, because the
USER() value never contains
wildcards, whereas account values (as returned by
CURRENT_USER()) may contain user
name and host name wildcards.
For example, a blank user name matches any user, so an account of
''@'localhost' enables clients to connect as an
anonymous user from the local host with any user name. If this
case, if a client connects as user1 from the
local host, USER() and
CURRENT_USER() return different
values:
mysql> SELECT USER(), CURRENT_USER();
+-----------------+----------------+
| USER() | CURRENT_USER() |
+-----------------+----------------+
| user1@localhost | @localhost |
+-----------------+----------------+
The host name part of an account can contain wildcards, too. If
the host name contains a '%' or
'_' pattern character or uses netmask notation,
the account can be used for clients connecting from multiple hosts
and the CURRENT_USER() value will
not indicate which one. For example, the account
'user2'@'%.example.com' can be used by
user2 to connect from any host in the
example.com domain. If user2
connects from remote.example.com,
USER() and
CURRENT_USER() return different
values:
mysql> SELECT USER(), CURRENT_USER();
+--------------------------+---------------------+
| USER() | CURRENT_USER() |
+--------------------------+---------------------+
| user2@remote.example.com | user2@%.example.com |
+--------------------------+---------------------+
If an application invokes USER()
for user auditing, but must also be able to associate the
USER() value with an account in the
user table, it is necessary to avoid accounts
that contain wildcards in the User or
Host column. Specifically, do not permit
User to be empty (which creates an
anonymous-user account), and do not permit pattern characters or
netmask notation in Host values. All accounts
must have a nonempty User value and literal
Host value.
With respect to the previous examples, the
''@'localhost' and
'user2'@'%.example.com' accounts should be
changed not to use wildcards:
RENAME USER ''@'localhost' TO 'user1'@'localhost'; RENAME USER 'user2'@'%.example.com' TO 'user2'@'remote.example.com';
If user2 must be able to connect from several
hosts in the example.com domain, there should
be a separate account for each host.
To extract the user name or host name part from a
CURRENT_USER() or
USER() value, use the
SUBSTRING() function:
mysql>SELECT SUBSTRING_INDEX(CURRENT_USER(),'@',1);+---------------------------------------+ | SUBSTRING_INDEX(CURRENT_USER(),'@',1) | +---------------------------------------+ | user1 | +---------------------------------------+ mysql>SELECT SUBSTRING_INDEX(CURRENT_USER(),'@',-1);+----------------------------------------+ | SUBSTRING_INDEX(CURRENT_USER(),'@',-1) | +----------------------------------------+ | localhost | +----------------------------------------+
In some cases, you might want to run multiple mysqld servers on the same machine. You might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might want to give different users access to different mysqld servers that they manage themselves. (For example, you might be an Internet Service Provider that wants to provide independent MySQL installations for different customers.)
To run multiple servers on a single machine, each server must have unique values for several operating parameters. These can be set on the command line or in option files. See Section 4.2.3, “Specifying Program Options”.
At least the following options must be different for each server:
--port controls the port number
for TCP/IP connections. (Alternatively, if the host has multiple
network addresses, you can use
--bind-address to cause different
servers to listen to different interfaces.)
--socket controls the Unix socket
file path on Unix and the name of the named pipe on Windows. On
Windows, it is necessary to specify distinct pipe names only for
those servers that support named-pipe connections.
--shared-memory-base-name=
name
The name of shared memory to use for shared-memory connections.
This option is available only on Windows. The default name is
MYSQL. The name is case sensitive. This
option was added in MySQL 4.1.
This option is used only on Unix. It indicates the path name of the file in which the server writes its process ID.
If you use the following log file options, they must be different for each server:
Section 5.3.6, “Server Log Maintenance”, discusses the log file options further.
For better performance, you can specify the following options differently for each server, to spread the load between several physical disks:
Having different temporary directories also makes it easier to determine which MySQL server created any given temporary file.
With very limited exceptions, each server should use a different
data directory, which is specified using the
--datadir=
option.
path
Normally, you should never have two servers that update data in
the same databases. This may lead to unpleasant surprises if your
operating system does not support fault-free system locking. If
(despite this warning) you run multiple servers using the same
data directory and they have logging enabled, you must use the
appropriate options to specify log file names that are unique to
each server. Otherwise, the servers try to log to the same files.
Please note that this kind of setup only works with
ISAM, MyISAM and
MERGE tables, and not with any of the other
storage engines.
The warning against sharing a data directory among servers also applies in an NFS environment. Permitting multiple MySQL servers to access a common data directory over NFS is a very bad idea.
The primary problem is that NFS is the speed bottleneck. It is not meant for such use.
Another risk with NFS is that you must devise a way to ensure that two or more servers do not interfere with each other. Usually NFS file locking is handled by the lockd daemon, but at the moment there is no platform that performs locking 100% reliably in every situation.
Make it easy for yourself: Forget about sharing a data directory among servers over NFS. A better solution is to have one computer that contains several CPUs and use an operating system that handles threads efficiently.
If you have multiple MySQL installations in different locations, you
can specify the base installation directory for each server with the
--basedir=
option to cause each server to use a different data directory, log
files, and PID file. (The defaults for all these values are
determined relative to the base directory). In that case, the only
other options you need to specify are the
path--socket and
--port options. Suppose that you
install different versions of MySQL using tarfile binary
distributions. These install in different locations, so you can
start the server for each installation using
bin/mysqld_safe under its own corresponding base
directory. mysqld_safe determines the proper
--basedir option to pass to
mysqld, and you need specify only the
--socket and
--port options to
mysqld_safe. (For versions of MySQL older than
4.0, use safe_mysqld rather than
mysqld_safe.)
As discussed in the following sections, it is possible to start
additional servers by setting environment variables or by specifying
appropriate command-line options. However, if you need to run
multiple servers on a more permanent basis, it is more convenient to
use option files to specify for each server those option values that
must be unique to it. The
--defaults-file option is useful for
this purpose.
You can run multiple servers on Windows by starting them manually from the command line, each with appropriate operating parameters. On Windows NT-based systems, you also have the option of installing several servers as Windows services and running them that way. General instructions for running MySQL servers from the command line or as services are given in Section 2.3, “Installing MySQL on Microsoft Windows”. This section describes how to make sure that you start each server with different values for those startup options that must be unique per server, such as the data directory. These options are described in Section 5.7, “Running Multiple MySQL Servers on the Same Machine”.
To start multiple servers manually from the command line, you
can specify the appropriate options on the command line or in an
option file. It is more convenient to place the options in an
option file, but it is necessary to make sure that each server
gets its own set of options. To do this, create an option file
for each server and tell the server the file name with a
--defaults-file option when you
run it.
Suppose that you want to run mysqld on port
3307 with a data directory of C:\mydata1,
and mysqld-max on port 3308 with a data
directory of C:\mydata2. (To do this, make
sure that before you start the servers, each data directory
exists and has its own copy of the mysql
database that contains the grant tables.) Then create two option
files. For example, create one file named
C:\my-opts1.cnf that looks like this:
[mysqld] datadir = C:/mydata1 port = 3307
Create a second file named C:\my-opts2.cnf
that looks like this:
[mysqld] datadir = C:/mydata2 port = 3308
Then start each server with its own option file:
C:\>C:\mysql\bin\mysqld --defaults-file=C:\my-opts1.cnfC:\>C:\mysql\bin\mysqld-max --defaults-file=C:\my-opts2.cnf
On NT, each server starts in the foreground (no new prompt appears until the server exits later), so you will need to issue those two commands in separate console windows.
To shut down the servers, you must connect to each using the appropriate port number:
C:\>C:\mysql\bin\mysqladmin --port=3307 shutdownC:\>C:\mysql\bin\mysqladmin --port=3308 shutdown
Servers configured as just described permit clients to connect
over TCP/IP. If your version of Windows supports named pipes and
you also want to permit named-pipe connections, use the
mysqld-nt or mysqld-max-nt
servers and specify options that enable the named pipe and
specify its name. Each server that supports named-pipe
connections must use a unique pipe name. For example, the
C:\my-opts1.cnf file might be written like
this:
[mysqld] datadir = C:/mydata1 port = 3307 enable-named-pipe socket = mypipe1
Then start the server this way:
C:\> C:\mysql\bin\mysqld-nt --defaults-file=C:\my-opts1.cnf
Modify C:\my-opts2.cnf similarly for use by
the second server.
A similar procedure applies for servers that you want to support
shared-memory connections. Enable such connections with the
--shared-memory option and
specify a unique shared-memory name for each server with the
--shared-memory-base-name option.
On NT-based systems, a MySQL server can run as a Windows service. The procedures for installing, controlling, and removing a single MySQL service are described in Section 2.3.11, “Starting MySQL as a Windows Service”.
As of MySQL 4.0.2, you can install multiple servers as services. In this case, you must make sure that each server uses a different service name in addition to all the other parameters that must be unique per server.
For the following instructions, assume that you want to run the
mysqld-nt server from two different versions
of MySQL that are installed at
C:\mysql-4.0.8 and
C:\mysql-4.0.17, respectively. (This might
be the case if you are running 4.0.8 as your production server,
but want to test 4.0.17 before upgrading to it.)
The following principles apply when installing a MySQL service
with the --install or
--install-manual option:
If you specify no service name, the server uses the default
service name of MySQL and the server
reads options from the [mysqld] group in
the standard option files.
If you specify a service name after the
--install option, the server ignores the
[mysqld] option group and instead reads
options from the group that has the same name as the
service. The server reads options from the standard option
files.
If you specify a
--defaults-file option after
the service name, the server ignores the standard option
files and reads options only from the
[mysqld] group of the named file.
Before MySQL 4.0.17, only a server installed using the default
service name (MySQL) or one installed
explicitly with a service name of mysqld
will read the [mysqld] group in the
standard option files. As of 4.0.17, all servers read the
[mysqld] group if they read the standard
option files, even if they are installed using another service
name. This enables you to use the [mysqld]
group for options that should be used by all MySQL services,
and an option group named after each service for use by the
server installed with that service name.
Based on the preceding information, you have several ways to set up multiple services. The following instructions describe some examples. Before trying any of them, be sure that you shut down and remove any existing MySQL services first.
Approach 1: Specify the
options for all services in one of the standard option
files. To do this, use a different service name for each
server. Suppose that you want to run the 4.0.8
mysqld-nt using the service name of
mysqld1 and the 4.0.17
mysqld-nt using the service name
mysqld2. In this case, you can use the
[mysqld1] group for 4.0.8 and the
[mysqld2] group for 4.0.17. For example,
you can set up C:\my.cnf like this:
# options for mysqld1 service [mysqld1] basedir = C:/mysql-4.0.8 port = 3307 enable-named-pipe socket = mypipe1 # options for mysqld2 service [mysqld2] basedir = C:/mysql-4.0.17 port = 3308 enable-named-pipe socket = mypipe2
Install the services as follows, using the full server path names to ensure that Windows registers the correct executable program for each service:
C:\>C:\mysql-4.0.8\bin\mysqld-nt --install mysqld1C:\>C:\mysql-4.0.17\bin\mysqld-nt --install mysqld2
To start the services, use the services manager, or use NET START with the appropriate service names:
C:\>NET START mysqld1C:\>NET START mysqld2
To stop the services, use the services manager, or use NET STOP with the appropriate service names:
C:\>NET STOP mysqld1C:\>NET STOP mysqld2
Approach 2: Specify options
for each server in separate files and use
--defaults-file when you
install the services to tell each server what file to use.
In this case, each file should list options using a
[mysqld] group.
With this approach, to specify options for the 4.0.8
mysqld-nt, create a file
C:\my-opts1.cnf that looks like this:
[mysqld] basedir = C:/mysql-4.0.8 port = 3307 enable-named-pipe socket = mypipe1
For the 4.0.17 mysqld-nt, create a file
C:\my-opts2.cnf that looks like this:
[mysqld] basedir = C:/mysql-4.0.17 port = 3308 enable-named-pipe socket = mypipe2
Install the services as follows (enter each command on a single line):
C:\>C:\mysql-4.0.8\bin\mysqld-nt --install mysqld1--defaults-file=C:\my-opts1.cnfC:\>C:\mysql-4.0.17\bin\mysqld-nt --install mysqld2--defaults-file=C:\my-opts2.cnf
To use a --defaults-file
option when you install a MySQL server as a service, you
must precede the option with the service name.
After installing the services, start and stop them the same way as in the preceding example.
To remove multiple services, use mysqld
--remove for each one, specifying a service name
following the --remove option. If the service
name is the default (MySQL), you can omit it.
The easiest way is to run multiple servers on Unix is to compile them with different TCP/IP ports and Unix socket files so that each one is listening on different network interfaces. Compiling in different base directories for each installation also results automatically in a separate, compiled-in data directory, log file, and PID file location for each server.
Assume that an existing server is configured for the default
TCP/IP port number (3306) and Unix socket file
(/tmp/mysql.sock). To configure a new server
to have different operating parameters, use a
configure command something like this:
shell>./configure --with-tcp-port=port_number\--with-unix-socket-path=file_name\--prefix=/usr/local/mysql-4.0.17
Here, port_number and
file_name must be different from the
default TCP/IP port number and Unix socket file path name, and the
--prefix value should specify an
installation directory different from the one under which the
existing MySQL installation is located.
If you have a MySQL server listening on a given port number, you can use the following command to find out what operating parameters it is using for several important configurable variables, including the base directory and Unix socket file name:
shell> mysqladmin --host=host_name --port=port_number variables
With the information displayed by that command, you can tell what option values not to use when configuring an additional server.
Note that if you specify localhost as a host
name, mysqladmin defaults to using a Unix
socket file connection rather than TCP/IP. In MySQL 4.1, you can
explicitly specify the connection protocol to use by using the
--protocol={TCP|SOCKET|PIPE|MEMORY}
option.
You do not have to compile a new MySQL server just to start with a different Unix socket file and TCP/IP port number. It is also possible to use the same server binary and start each invocation of it with different parameter values at runtime. One way to do so is by using command-line options:
shell> mysqld_safe --socket=file_name --port=port_number
To start a second server, provide different
--socket and
--port option values, and pass a
--datadir=
option to mysqld_safe so that the server uses a
different data directory.
path
Another way to achieve a similar effect is to use environment variables to set the Unix socket file name and TCP/IP port number:
shell>MYSQL_UNIX_PORT=/tmp/mysqld-new.sockshell>MYSQL_TCP_PORT=3307shell>export MYSQL_UNIX_PORT MYSQL_TCP_PORTshell>mysql_install_db --user=mysqlshell>mysqld_safe --datadir=/path/to/datadir &
This is a quick way of starting a second server to use for testing. The nice thing about this method is that the environment variable settings apply to any client programs that you invoke from the same shell. Thus, connections for those clients are automatically directed to the second server.
Section 2.13, “Environment Variables”, includes a list of other environment variables you can use to affect mysqld.
For automatic server execution, the startup script that is executed at boot time should run the following command once for each server with an appropriate option file path for each command:
shell> mysqld_safe --defaults-file=file_name
Each option file should contain option values specific to a given server.
On Unix, the mysqld_multi script is another way to start multiple servers. See Section 4.3.4, “mysqld_multi — Manage Multiple MySQL Servers”.
To connect with a client program to a MySQL server that is listening to different network interfaces from those compiled into your client, you can use one of the following methods:
Start the client with
--host=
host_name--port=
to connect using TCP/IP to a remote server, with
port_number--host=127.0.0.1
--port=
to connect using TCP/IP to a local server, or with
port_number--host=localhost
--socket=
to connect to a local server using a Unix socket file or a
Windows named pipe.
file_name
As of MySQL 4.1, start the client with
--protocol=TCP to connect
using TCP/IP,
--protocol=SOCKET to connect
using a Unix socket file,
--protocol=PIPE to connect
using a named pipe, or
--protocol=MEMORY to connect
using shared memory. For TCP/IP connections, you may also need
to specify --host and
--port options. For the other
types of connections, you may need to specify a
--socket option to specify a
Unix socket file or Windows named-pipe name, or a
--shared-memory-base-name
option to specify the shared-memory name. Shared-memory
connections are supported only on Windows.
On Unix, set the MYSQL_UNIX_PORT and
MYSQL_TCP_PORT environment variables to
point to the Unix socket file and TCP/IP port number before
you start your clients. If you normally use a specific socket
file or port number, you can place commands to set these
environment variables in your .login file
so that they apply each time you log in. See
Section 2.13, “Environment Variables”.
Specify the default Unix socket file and TCP/IP port number in
the [client] group of an option file. For
example, you can use C:\my.cnf on
Windows, or the .my.cnf file in your home
directory on Unix. See Section 4.2.3.3, “Using Option Files”.
In a C program, you can specify the socket file or port number
arguments in the
mysql_real_connect() call. You
can also have the program read option files by calling
mysql_options(). See
Section 17.6.6, “C API Function Descriptions”.
If you are using the Perl DBD::mysql
module, you can read options from MySQL option files. For
example:
$dsn = "DBI:mysql:test;mysql_read_default_group=client;"
. "mysql_read_default_file=/usr/local/mysql/data/my.cnf";
$dbh = DBI->connect($dsn, $user, $password);
See Section 17.8, “MySQL Perl API”.
Other programming interfaces may provide similar capabilities for reading option files.