mysqld - the MySQL server
mysqld [options]
mysqld is the MySQL server. The following discussion covers these MySQL server configuration topics:
o Startup options that the server supports o Server system variables o Server status variables o How to set the server SQL mode o The server shutdown process
When you start the mysqld server, you can specify program options using any of the methods described in Section 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 3.2, 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 [xxxxx_SERVER] groups, where 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:
As of MySQL 4.0, you can assign a value to a server system variable by using an option of the form --var_name=value. For example, --key_buffer_size=32M sets the key_buffer_size variable to a value of 32MB.
o Options that affect security: See Section 5.3, Security-Related mysqld Options. o SSL-related options: See Section 7.7.4, SSL Command Options. o Binary log control options: See Section 10.4, The Binary Log. o Replication-related options: See Section 8, Replication Startup Options. o Options specific to particular storage engines: See Section 1.1, MyISAM Startup Options, Section 5.3, BDB Startup Options, Section 2.5, InnoDB Startup Options and System Variables, and Section 5.5.1, MySQL Cluster-Related Command Options for mysqld. You can also set the values of server system variables by using variable names as options, as described later in this section. o --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.
o --allow-suspicious-udfs 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 2.4.6, User-Defined Function Security Precautions.
o --ansi 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 9.3, Running MySQL in ANSI Mode, and the section called THE SERVER SQL MODE.
o --basedir=path, -b path The path to the MySQL installation directory. All paths are usually resolved relative to this directory.
o --big-tables Allow 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.
o --bind-address=IP The IP address to bind to.
o --bootstrap This option is used by the mysql_install_db script to create the MySQL privilege tables without having to start a full MySQL server.
o --character-sets-dir=path The directory where character sets are installed. See Section 9.1, The Character Set Used for Data and Sorting.
o --character-set-client-handshake Don’t 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.
o --character-set-server=charset_name, -C charset_name Use charset_name as the default server character set. This option is available as of MySQL 4.1.3. See Section 9.1, The Character Set Used for Data and Sorting.
o --chroot=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.
o --collation-server=collation_name Use collation_name as the default server collation. This option is available as of MySQL 4.1.3. See Section 9.1, The Character Set Used for Data and Sorting.
o --console (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.
o --core-file Write a core file if mysqld dies. For some systems, you must also specify the --core-file-size option to mysqld_safe. See mysqld_safe(1). Note that on some systems, such as Solaris, you do not get a core file if you are also using the --user option.
o --datadir=path, -h path The path to the data directory.
o --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. The debug_options string often is 'd:t:o,file_name’. The default is 'd:t:i:o,mysqld.trace’. See Section 1.2, Creating Trace Files.
o --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 9.1, The Character Set Used for Data and Sorting.
o --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 9.1, The Character Set Used for Data and Sorting.
o --default-storage-engine=type This option is a synonym for --default-table-type. It is available as of MySQL 4.1.2.
o --default-table-type=type Set the default table type (storage engine) for tables. See Chapter 14, Storage Engines and Table Types.
o --default-time-zone=timezone 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.
o --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 5.2, Tuning Server Parameters, and Section 1.1, MyISAM Startup Options.
Note: 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.
o --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 instead.
o --des-key-file=file_name Read the default DES keys from this file. These keys are used by the DES_ENCRYPT() and DES_DECRYPT() functions.
o --enable-named-pipe 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.
o --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!
o --external-locking 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.
Note: If you use this option to enable updates to MyISAM tables from many MySQL processes, you must ensure that the following conditions are satisfied:
o You should not use the query cache for queries that use tables that are updated by another process. o You should not use --delay-key-write=ALL or DELAY_KEY_WRITE=1 on any shared tables. The easiest way to ensure this is to always use --external-locking together with --delay-key-write=OFF and --query-cache-size=0. (This is not done by default because in many setups it is useful to have a mixture of the preceding options.) o --flush 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 4.2, What to Do If MySQL Keeps Crashing.
o --init-file=file_name Read SQL statements from this file at startup. Each statement must be on a single line and should not include comments.
o --innodb-safe-binlog Adds consistency guarantees between the content of InnoDB tables and the binary log. See Section 10.4, The Binary Log.
o --innodb-xxx The InnoDB options are listed in Section 2.5, InnoDB Startup Options and System Variables.
o --language=lang_name, -L lang_name Return client error messages in the given language. lang_name can be given as the language name or as the full pathname to the directory where the language files are installed. See Section 9.2, Setting the Error Message Language.
o --log[=file_name], -l [file_name] Log connections and SQL statements received from clients to this file. See Section 10.2, The General Query Log. If you omit the filename, MySQL uses host_name.log as the filename.
o --log-bin[=base_name] Enable binary logging. The server logs all statements that change data to the binary log, which is used for backup and replication. See Section 10.4, The Binary Log.
The option value, if given, is the basename for the log sequence. The server creates binary log files in sequence by adding a numeric suffix to the basename. It is recommended that you specify a basename (see Section 8.4, Open Issues in MySQL, for the reason). Otherwise, MySQL uses host_name-bin as the basename.
o --log-bin-index[=file_name] The index file for binary log filenames. See Section 10.4, The Binary Log. If you omit the filename, and if you didn’t specify one with --log-bin, MySQL uses host_name-bin.index as the filename.
o --log-error[=file_name] Log errors and startup messages to this file. See Section 10.1, The Error Log. If you omit the filename, MySQL uses host_name.err. If the filename has no extension, the server adds an extension of .err.
o --log-isam[=file_name] Log all ISAM/MyISAM changes to this file (used only when debugging ISAM/MyISAM).
o --log-long-format Log extra information to the update log, binary update log, and slow query log, if they have been activated. For example, the username 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.
o --log-queries-not-using-indexes If you are using this option with --log-slow-queries, queries that do not use indexes also are logged to the slow query log. This option is available as of MySQL 4.1. See Section 10.5, The Slow Query Log.
o --log-short-format Log less information to the update log, binary update log, and slow query log, if they have been activated. For example, the username and timestamp are not logged for queries. This option was introduced in MySQL 4.1.
o --log-slow-admin-statements 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.)
o --log-slow-queries[=file_name] Log all queries that have taken more than long_query_time seconds to execute to this file. See Section 10.5, 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.
o --log-update[=file_name] Log updates to fileN where N is a unique number if not given. See Section 10.3, The Update Log. The update log is now deprecated; you should use the binary log instead (--log-bin). See Section 10.4, The Binary Log.
o --log-warnings[=level], -W [level] Print out warnings such as Aborted connection... to the error log. Enabling this option 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. Aborted connections are not logged to the error log unless the value is greater than 1. See Section 2.10, Communication Errors and Aborted Connections.
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.
o --low-priority-updates Give table-modifying operations (INSERT, REPLACE, DELETE, UPDATE) lower priority than selects. This can also be done via {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. See Section 3.2, Table Locking Issues.
o --memlock Lock the mysqld process in memory. This works on systems such as Solaris that support the mlockall() system call. This might help if you have a problem where the operating system is causing mysqld to swap on disk. Note that use of this option requires that you run the server as root, which is normally not a good idea for security reasons. See Section 5.5, How to Run MySQL as a Normal User.
o --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 wasn’t 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: OptionDescriptionDEFAULTThe same as not giving any option to --myisam-recover.BACKUPIf the data file was changed during recovery, save a backup of the
tbl_name.MYD
file as
tbl_name-datetime.BAK.FORCERun recovery even if we would lose more than one row from the
.MYD file.QUICKdo 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.This option is available as of MySQL 3.23.25.
o --ndb-connectstring=connect_string When using the NDB storage engine, it is possible to point out the management server that distributes the cluster configuration by setting the connect string option. See Section 4.4.2, The MySQL Cluster connectstring, for syntax.
o --ndbcluster If the binary includes support for the NDB Cluster storage engine (from version 4.1.3, the MySQL-Max binaries are built with NDB Cluster enabled), this option enables the engine, which is disabled by default. Using the NDB Cluster storage engine is necessary for using MySQL Cluster. See Chapter 15, MySQL Cluster.
o --new 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:
o Hexadecimal strings such as 0xFF are treated as strings by default rather than as numbers. (Works in 4.0.12 and up.) o TIMESTAMP is returned as a string with the format 'YYYY-MM-DD HH:MM:SS’. (Works in 4.0.13 and up.) See Chapter 11, 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. o --old-passwords 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 6.9, Password Hashing as of MySQL 4.1.
o --old-protocol, -o Use the 3.20 protocol for compatibility with some very old clients.
o --one-thread Only use one thread (for debugging under Linux). This option is available only if the server is built with debugging enabled. See Section 1, Debugging a MySQL Server.
o --open-files-limit=count Change the number of file descriptors available to mysqld. If this option is not set or is set to 0, mysqld uses the value to reserve file descriptors with setrlimit(). If the value is 0, mysqld reserves max_connections×5 or max_connections + table_open_cache×2 files (whichever is larger). You should try increasing this value if mysqld gives you the error Too many open files.
o --pid-file=path The pathname of the process ID file. This file is used by other programs such as mysqld_safe to determine the server’s process ID.
o --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.
o --safe-mode Skip some optimization stages.
o --safe-show-database 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 6.3, Privileges Provided by MySQL.
o --safe-user-create If this option is enabled, a user cannot create new MySQL users by using the GRANT statement, if the user doesn’t have the INSERT privilege for the mysql.user table or any column in the table.
o --secure-auth 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.
o --shared-memory Enable shared-memory connections by local clients. This option is available only on Windows. It was added in MySQL 4.1.0.
o --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.
o --skip-bdb Disable the BDB storage engine. This saves memory and might speed up some operations. Do not use this option if you require BDB tables.
o --skip-concurrent-insert 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 3.3, Concurrent Inserts.
o --skip-delay-key-write 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 5.2, Tuning Server Parameters.
o --skip-external-locking Do not use external locking (system locking). With external locking disabled, you must shut down the server to use myisamchk or isamchk. See Section 4.3, MySQL Stability. As of MySQL 3.23, you can use the CHECK TABLE and REPAIR TABLE statements to check and repair MyISAM tables. This option previously was named --skip-locking.
External locking has been disabled by default since MySQL 4.0.
o --skip-grant-tables This option causes the server not to use 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).
o --skip-host-cache Do not use the internal hostname cache for faster name-to-IP resolution. Instead, query the DNS server every time a client connects. See Section 5.5, How MySQL Uses DNS.
o --skip-innodb Disable the InnoDB storage engine. This saves memory and disk space and might speed up some operations. Do not use this option if you require InnoDB tables.
o --skip-isam 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.
o --skip-name-resolve Do not resolve hostnames when checking client connections. Use only IP numbers. If you use this option, all Host column values in the grant tables must be IP numbers or localhost. See Section 5.5, How MySQL Uses DNS.
o --skip-ndbcluster Disable the NDB Cluster storage engine. This is the default for binaries that were built with NDB Cluster storage engine support; the server allocates memory and other resources for this storage engine only if the --ndbcluster option is given explicitly. See Section 4.3, Quick Test Setup of MySQL Cluster, for an example of usage.
o --skip-networking Do not listen for TCP/IP connections at all. All interaction with mysqld must be made via 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 allowed. See Section 5.5, How MySQL Uses DNS.
o --skip-new do not use new, possibly wrong routines.
o --skip-symlink This is the old form of --skip-symbolic-links, for use before MySQL 4.0.13.
o --standalone Available on Windows NT-based systems only; instructs the MySQL server not to run as a service.
o --symbolic-links, --skip-symbolic-links Enable or disable symbolic link support. This option has different effects on Windows and Unix:
o On Windows, enabling symbolic links allows you to establish a symbolic link to a database directory by creating a db_name.sym file that contains the path to the real directory. See Section 6.1.3, Using Symbolic Links for Databases on Windows. o 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 6.1.2, Using Symbolic Links for Tables on Unix. This option was added in MySQL 4.0.13. o --skip-safemalloc 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.
o --skip-show-database With this option, the SHOW DATABASES statement is allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. Without this option, SHOW DATABASES is allowed 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.
o --skip-stack-trace 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 1, Debugging a MySQL Server.
o --skip-thread-priority Disable using thread priorities for faster response time.
o --socket=path On Unix, this option specifies the Unix socket file to use when listening for local connections. The default value is /tmp/mysql.sock. 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).
o --sql-mode=value[,value[,value...]] Set the SQL mode. See the section called THE SERVER SQL MODE. This option was added in 3.23.41.
o --temp-pool 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.
o --transaction-isolation=level Sets the default transaction isolation level. The level value can be READ-UNCOMMITTED, READ-COMMITTED, REPEATABLE-READ, or SERIALIZABLE. See Section 4.6, SET TRANSACTION Syntax.
o --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 filesystem or to a directory that is cleared when the server host restarts. For more information about the storage location of temporary files, see Section 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.
o --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 5.1, 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.
o --version, -V Display version information and exit.
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 allowable value if only certain values are allowed.
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-var_name=value command-line option.
It is also possible to set variables by using --set-variable=var_name=value or --var_name=value syntax. This syntax is deprecated as of MySQL 4.0.
You can change the values of most system variables for a running server with the SET statement. See Section 5.3, SET Syntax.
the section called SERVER SYSTEM VARIABLES, provides a full description for all variables, and additional information for setting them at server startup and runtime. Section 5.2, 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:
o 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 --helpo 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 --helpo 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. For additional system variable information, see these sections: o the section called USING SYSTEM VARIABLES, discusses the syntax for setting and displaying system variable values. o the section called Dynamic System Variables, lists the variables that can be set at runtime. o Information on tuning sytem variables can be found in Section 5.2, Tuning Server Parameters. o Section 2.5, InnoDB Startup Options and System Variables, lists InnoDB system variables. Note: 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. Values for buffer sizes, lengths, and stack sizes are given in bytes unless otherwise specified. o ansi_mode This is ON if mysqld was started with --ansi. See Section 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.
o back_log 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.
o basedir The MySQL installation base directory. This variable can be set with the --basedir option.
o bdb_cache_size 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.
o bdb_home 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.
o bdb_log_buffer_size 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.
o bdb_logdir 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.
o bdb_max_lock 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.o bdb_shared_data 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.
o bdb_tmpdir The BDB temporary file directory. This variable was added in MySQL 3.23.14.
o bdb_version See the description for version_bdb.
o binlog_cache_size 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 10.4, The Binary Log.
o bulk_insert_buffer_size 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 non-empty 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. This variable was added in MySQL 4.0.3. This variable previously was named myisam_bulk_insert_tree_size.
o character_set 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_xxx variables.
o character_set_client The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.
o character_set_connection 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.
o character_set_database 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.
o character_set_results The character set used for returning query results to the client. This variable was added in MySQL 4.1.1.
o character_set_server The server default character set. This variable was added in MySQL 4.1.1.
o character_set_system The character set used by the server for storing identifiers. The value is always utf8. This variable was added in MySQL 4.1.1.
o character_sets 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.)
o character_sets_dir The directory where character sets are installed. This variable was added in MySQL 4.1.2.
o collation_connection The collation of the connection character set. This variable was added in MySQL 4.1.1.
o collation_database 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.
o collation_server The server default collation. This variable was added in MySQL 4.1.1.
o concurrent_insert If ON (the default), MySQL allows INSERT and SELECT statements to run concurrently for MyISAM tables that have no free blocks in the middle. You can turn this option off by starting mysqld with --safe or --skip-new. This variable was added in MySQL 3.23.7.
See also Section 3.3, Concurrent Inserts.
o connect_timeout The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake.
o convert_character_set The current character set mapping that was set by SET CHARACTER SET. This variable was removed in MySQL 4.1.
o datadir The MySQL data directory. This variable can be set with the --datadir option.
o date_format This variable is not implemented.
o datetime_format This variable is not implemented.
o default_week_format The default mode value to use for the WEEK() function. See Section 5, Date and Time Functions. This variable is available as of MySQL 4.0.14.
o delay_key_write 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. OptionDescriptionOFFDELAY_KEY_WRITE is ignored.ONMySQL honors any DELAY_KEY_WRITE option specified in
CREATE TABLE statements. This
is the default value.ALLAll 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 the section called \FBMYSQLD\FR COMMAND OPTIONS, and Section 1.1, MyISAM Startup Options.Note that enabling external locking with --external-locking offers no protection against index corruption for tables that use delayed key writes.
This variable was added in MySQL 3.23.8.
o delayed_insert_limit After inserting delayed_insert_limit delayed rows, the INSERT DELAYED handler thread checks whether there are any SELECT statements pending. If so, it allows them to execute before continuing to insert delayed rows.
o delayed_insert_timeout How many seconds an INSERT DELAYED handler thread should wait for INSERT statements before terminating.
o delayed_queue_size 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.
o expire_logs_days The number of days for automatic binary log removal. The default is 0, which means no automatic removal. Possible removals happen at startup and at binary log rotation. This variable was added in MySQL 4.1.0.
o flush 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 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.
o flush_time If this is set to a non-zero value, all tables are closed every flush_time seconds to free up resources and synchronize unflushed data to disk. We recommend that this option be used only on Windows 9x or Me, or on systems with minimal resources. This variable was added in MySQL 3.22.18.
o ft_boolean_syntax The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 7.1, 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:
o Operator function is determined by position within the string. o The replacement value must be 14 characters. o Each character must be an ASCII non-alphanumeric character. o Either the first or second character must be a space. o No duplicates are allowed 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. o Positions 10, 13, and 14 (which by default are set to :, &, and |) are reserved for future extensions. o ft_max_word_len The maximum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.
Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
o ft_min_word_len The minimum length of the word to be included in a FULLTEXT index. This variable was added in MySQL 4.0.0.
Note: FULLTEXT indexes must be rebuilt after changing this variable. Use REPAIR TABLE tbl_name QUICK.
o ft_query_expansion_limit 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.
o ft_stopword_file The file from which to read the list of stopwords for full-text searches. 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. This variable was added in MySQL 4.0.10.
Note: FULLTEXT indexes must be rebuilt after changing this variable or the contents of the stopword file. Use REPAIR TABLE tbl_name QUICK.
o group_concat_max_len The maximum allowed result length for the GROUP_CONCAT() function. The default is 1024. This variable was added in MySQL 4.1.0.
o have_archive YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.3.
o have_bdb YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.
o have_blackhole_engine YES if mysqld supports BLACKHOLE tables, NO if not. This variable was added in MySQL 4.1.11.
o have_compress 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.
o have_crypt 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.
o have_csv YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.4.
o have_example_engine YES if mysqld supports EXAMPLE tables, NO if not. This variable was added in MySQL 4.1.4.
o have_geometry YES if the server supports spatial data types, NO if not. This variable was added in MySQL 4.1.3.
o have_innodb YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.
o have_isam YES if mysqld supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.
o have_ndbcluster YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used. This variable was added in MySQL 4.1.2.
o have_openssl YES if mysqld supports SSL (encryption) connections, NO if not. This variable was added in MySQL 3.23.43.
o have_query_cache YES if mysqld supports the query cache, NO if not. This variable was added in MySQL 4.0.2.
o have_raid YES if mysqld supports the RAID option, NO if not. This variable was added in MySQL 3.23.30.
o have_rtree_keys 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.
o have_symlink 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.
o init_connect 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’Note that 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.
o init_file 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. This variable was added in MySQL 3.23.2.
o init_slave This variable is similar to init_connect, but is a string to be executed by a slave server each time the SQL thread starts. The format of the string is the same as for the init_connect variable. This variable was added in MySQL 4.1.2.
o innodb_xxx InnoDB system variables are listed in Section 2.5, InnoDB Startup Options and System Variables.
o interactive_timeout 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.
o join_buffer_size The size of the buffer that is used for 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.
o key_buffer_size 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 allowable 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.
Increase the value to get better index handling (for all reads and multiple writes) to as much as you can afford. Using a value that is 25% of total memory on a machine that mainly runs MySQL is quite common. However, if you make the value too large (for example, more than 50% of your total memory) your system might start to page and become extremely slow. MySQL relies on the operating system to perform filesystem caching for data reads, so you must leave some room for the filesystem cache. Consider also the memory requirements of other storage engines.
For even more speed when writing many rows at the same time, use LOCK TABLES. See Section 2.13, 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 5.4, 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 may be allocated internally for administrative structures.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_sizeHowever, Key_blocks_used indicates the maximum number of blocks that have ever been in use at once, so this formula does not necessary 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 4.6, The MyISAM Key Cache.
o key_cache_age_threshold This value controls the demotion of buffers from the hot sub-chain of a key cache to the warm sub-chain. 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 4.6, The MyISAM Key Cache.
o key_cache_block_size 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 4.6, The MyISAM Key Cache.
o key_cache_division_limit The division point between the hot and warm sub-chains of the key cache buffer chain. The value is the percentage of the buffer chain to use for the warm sub-chain. Allowable values range from 1 to 100. The default value is 100. This variable was added in MySQL 4.1.1. See Section 4.6, The MyISAM Key Cache.
o language The language used for error messages.
o large_file_support Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.
o large_pages Whether large page support is enabled. This variable was added in MySQL 5.0.3.
o license The type of license the server has. This variable was added in MySQL 4.0.19.
o local_infile Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.4, Security Issues with LOAD DATA LOCAL. This variable was added in MySQL 4.0.3.
o locked_in_memory Whether mysqld was locked in memory with --memlock. This variable was added in MySQL 3.23.25.
o log Whether logging of all statements to the general query log is enabled. See Section 10.2, The General Query Log.
o log_bin Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See Section 10.4, The Binary Log.
o log_error The location of the error log. This variable was added in MySQL 4.0.10.
o log_slave_updates Whether updates received by a slave server from a master server should be logged to the slave’s own binary log. Binary logging must be enabled on the slave for this variable to have any effect. This variable was added in MySQL 3.23.17. See Section 8, Replication Startup Options.
o log_slow_queries 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 10.5, The Slow Query Log.
o log_update 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 10.3, The Update Log.
o log_warnings 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.
o long_query_time 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 10.5, The Slow Query Log.
o low_priority_updates 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 variable previously was named sql_low_priority_updates. It was added in MySQL 3.22.5.
o lower_case_file_system This variable describes the case sensitivity of filenames on the filesystem where the data directory is located. OFF means filenames are case sensitive, ON means they are not case sensitive. This variable was added in MySQL 4.0.19.
o lower_case_table_names If set to 1 table names are stored in lowercase on disk and table name 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. See Section 2.2, Identifier Case Sensitivity.
Note: If you are using InnoDB tables, you should set this variable to 1 on all platforms to force names to be converted to lowercase.
You should not set this variable to 0 if you are running MySQL on a system that does not have case-sensitive filenames (such as Windows or Mac OS X). New in 4.0.18: If this variable is not set at startup and the filesystem on which the data directory is located does not have case-sensitive filenames, MySQL automatically sets lower_case_table_names to 2.
o max_allowed_packet 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.
o max_binlog_cache_size If a multiple-statement transaction requires more than this amount of memory, the server generates a Multi-statement transaction required more than ’max_binlog_cache_size’ bytes of storage error. This variable was added in MySQL 3.23.29.
o max_binlog_size If a write to the binary log causes the current log file size to exceed the value of this variable, the server rotates the binary logs (closes the current file and opens the next one). You cannot set this variable to more than 1GB or to less than 4096 bytes. (The minimum before MYSQL 4.0.14 is 1024 bytes.) The default value is 1GB. This variable was added in MySQL 3.23.33.
A transaction is written in one chunk to the binary log, so it is never split between several binary logs. Therefore, if you have big transactions, you might see binary logs larger than max_binlog_size.
If max_relay_log_size is 0, the value of max_binlog_size applies to relay logs as well. max_relay_log_size was added in MySQL 4.0.14.
o max_connect_errors 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.
o max_connections The number of simultaneous client connections allowed. Increasing this value increases the number of file descriptors that mysqld requires. See Section 4.8, How MySQL Opens and Closes Tables, for comments on file descriptor limits. See also Section 2.6, Too many connections.
o max_delayed_threads 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 wasn’t 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.
o max_error_count 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.
o max_heap_table_size This variable sets the maximum size to which MEMORY (HEAP) tables are allowed 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. This variable was added in MySQL 3.23.0.
o max_insert_delayed_threads This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.
o max_join_size Do not allow SELECT 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 SELECT 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.
o max_length_for_sort_data The cutoff on the size of index values that determines which filesort algorithm to use. See Section 2.9, ORDER BY Optimization. This variable was added in MySQL 4.1.1
o max_prepared_stmt_count 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. The default value is 16,382. The allowable range of values is from 0 to 1 milliion. 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. This variable was added in MySQL 4.1.19.
o max_relay_log_size 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 3, Replication Implementation Details.
o max_seeks_for_key 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 5.4.11, 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.
o max_sort_length 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.
o max_tmp_tables The maximum number of temporary tables a client can keep open at the same time. (This option does not yet do anything.)
o max_user_connections The maximum number of simultaneous connections allowed to any given MySQL account. A value of 0 means no limit. This variable was added in MySQL 3.23.34.
This variable has only a global form.
o max_write_lock_count After this many write locks, allow some pending read lock requests to be processed in between. This variable was added in MySQL 3.23.7.
o myisam_data_pointer_size 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 2.11, The table is full.
o 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. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
o myisam_max_sort_file_size The maximum size of the temporary file that MySQL is allowed 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. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.
o myisam_recover_options The value of the --myisam-recover option. See the section called \FBMYSQLD\FR COMMAND OPTIONS. This variable was added in MySQL 3.23.36.
o myisam_repair_threads 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.
Note: Multi-threaded repair is still beta-quality code. This variable was added in MySQL 4.0.13.
o myisam_sort_buffer_size 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.
o myisam_stats_method How the server treats NULL values when collecting statistics about the distribution of index values for MyISAM tables. This variable has two possible values, nulls_equal and nulls_unequal. 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.
The method that is used for generating table statistics influences how the optimizer chooses indexes for query execution, as described in Section 4.7, MyISAM Index Statistics Collection.
This variable was added in MySQL 4.1.15/5.0.14. For older versions, the statistics collection method is equivalent to nulls_equal.
o named_pipe On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.
o ndb_autoincrement_prefetch_sz Determines the probability of gaps in an autoincremented column. Set to 1 to minimize this. Set to a high value for optimization — makes inserts faster, but decreases the likelihood that consecutive autoincrement numbers will be used in a batch of inserts. Default value: 32. Mimimum value: 1.
o ndb_cache_check_time The number of seconds to wait before checking the NDB query cache. Setting this to 0 (the default and minimum value) means that the NDB query cache will be checked for validation upon each query. A large value means the NDB query cache will seldom be checked and invalidated because of updates on another mysqld. It is generally not desirable to set this to a value greater than 2.
o ndb_force_send Forces sending of buffers to NDB immediately, without waiting for other threads. Defaults to ON.
o ndb_index_stat_cache_entries Sets the granularity of the statistics by determining the number of starting and ending keys to store in the statistics memory cache. Zero means no caching takes place; in this case, the data nodes are always queries directly. Default value: 32.
o ndb_index_stat_enable Use NDB index statistics in query optimization. Defaults to ON.
o ndb_index_stat_update_freq How often to query data nodes instead of the statistics cache. For example, a value of 20 (the default) means to direct every 20th query to the data nodes.
o ndb_report_thresh_binlog_epoch_slip This is a threshold on the number of epochs to be behind before reporting binlog status. For example, a value of 3 (the default) means that if the difference between which epoch has been received from the storage nodes and which epoch has been applied to the binlog is 3 or more, a status message will be sent to the cluster log.
o ndb_report_thresh_binlog_mem_usage This is a threshold on the percentage of free memory remaining before reporting binlog status. For example, a value of 10 (the default) means that if the amount of available memory for receiving binlog data from the data nodes falls below 10%, a status message will be sent to the cluster log.
o ndb_use_exact_count Forces NDB to use an count of records during SELECT COUNT(*) query planning to speed up this type of query. The default value is ON. For faster queries overall, disable this feature by setting the value of ndb_use_exact_count to OFF.
o ndb_use_transactions You can disable NDB transaction support by setting this variable’s values to OFF (not recommended). The default is ON.
o net_buffer_length The communication buffer is reset to this size between SQL statements. 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 buffer is automatically enlarged, up to max_allowed_packet bytes.
o net_read_timeout 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 via 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.
o net_retry_count 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.
o net_write_timeout 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 via Unix socket files, named pipes, or shared memory. See also net_read_timeout. This variable was added in MySQL 3.23.20.
o new This variable is used in MySQL 4.0 to turn on some 4.1 behaviors. This variable was added in MySQL 4.0.12.
o old_passwords Whether the server should use pre-4.1-style passwords for MySQL user accounts. This variable was added in MySQL 4.1.1.
o one_shot This is not a variable, but it can be used when setting some variables. It is described in Section 5.3, SET Syntax.
o open_files_limit The number of files that the operating system allows mysqld to open. This is the real value allowed 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 can’t change the number of open files. This variable was added in MySQL 3.23.20.
o pid_file The pathname 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.
o port The number of the port on which the server listens for TCP/IP connections. This variable can be set with the --port option.
o preload_buffer_size The size of the buffer that is allocated when preloading indexes. This variable was added in MySQL 4.1.1.
o prepared_stmt_count 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.
o protocol_version The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.
o query_alloc_block_size 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 a bit. This variable was added in MySQL 4.0.16.
o query_cache_limit Don’t cache results that are larger than this number of bytes. The default value is 1MB. This variable was added in MySQL 4.0.1.
o query_cache_min_res_unit The minimum size for blocks allocated by the query cache. The default value is 4KB. Tuning information for this variable is given in Section 12.3, Query Cache Configuration. This variable is present from MySQL 4.1.
o query_cache_size The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. Note that query_cache_size bytes of memory are allocated even if if query_cache_type is set to 0. This variable was added in MySQL 4.0.1.
o query_cache_type 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. OptionDescription0 or OFFDon’t 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 ONCache all query results except for those that begin with SELECT
SQL_NO_CACHE.2 or DEMANDCache results only for queries that begin with SELECT
SQL_CACHE.This variable was added in MySQL 4.0.3.o query_cache_wlock_invalidate 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.
o query_prealloc_size 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.
o range_alloc_block_size The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.
o read_buffer_size Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you might want to increase this value. This variable was added in MySQL 4.0.3. Previously, it was named record_buffer.
o read_only When the variable is set to ON for a replication slave server, it causes the slave to allow no updates except from slave threads or from users that have the SUPER privilege. This can be useful to ensure that a slave server accepts updates only from its master server and not from clients. This variable was added in MySQL 4.0.14.
o relay_log_purge Disables or enables automatic purging of relay logs as soon as they are not needed any more. The default value is 1 (ON). This variable was added in MySQL 4.1.1.
o read_rnd_buffer_size When reading rows in sorted order following a key-sorting operation, the rows are read through this buffer to avoid disk seeks. 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. This variable was added in MySQL 4.0.3. Previously, it was named record_rnd_buffer.
o safe_show_database 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.
o secure_auth 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.
o server_id The server ID. This value is set by the --server-id option. It is used for replication to enable master and slave servers to identify themselves uniquely. This variable was added in MySQL 3.23.26.
o shared_memory (Windows only.) Whether the server allows shared-memory connections. This variable was added in MySQL 4.1.1.
o shared_memory_base_name (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.
o skip_external_locking This is OFF if mysqld uses external locking, ON if external locking is disabled. This variable was added in MySQL 4.0.3. Previously, it was named skip_locking.
o skip_networking This is ON if the server allows 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.
o skip_show_database 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 allowed only to users who have the SHOW DATABASES privilege, and the statement displays all database names. If the value is OFF, SHOW DATABASES is allowed 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.
o slave_compressed_protocol Whether to use compression of the master/slave protocol if both the slave and the master support it. This variable was added in MySQL 4.0.3.
o slave_load_tmpdir The name of the directory where the slave creates temporary files for replicating LOAD DATA INFILE statements. This variable was added in MySQL 4.0.0.
o slave_net_timeout The number of seconds to wait for more data from a master/slave connection before aborting the read. This timeout applies only to TCP/IP connections, not to connections made via Unix socket files, named pipes, or shared memory. This variable was added in MySQL 3.23.40.
o slave_skip_errors The replication errors that the slave should skip (ignore). This variable was added in MySQL 3.23.47.
o slave_transaction_retries If a replication slave SQL thread fails to execute a transaction because of an InnoDB deadlock or InnoDB’s innodb_lock_wait_timeout or NDB Cluster’s TransactionDeadlockDetectionTimeout or TransactionInactiveTimeout was exceeded, it automatically retries slave_transaction_retries times before stopping with an error. The default in MySQL 4.1 is 0. You must explicitly set the value to greater than 0 to enable the retry behavior, which is probably a good idea.
o slow_launch_time 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.
o socket 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).
o sort_buffer_size Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See Section 4.4, Where MySQL Stores Temporary Files.
o sql_mode 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 the section called THE SERVER SQL MODE.
o sql_slave_skip_counter The number of events from the master that a slave server should skip. See Section 6.2.6, SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax. This variable was added in MySQL 3.23.33.
o storage_engine This variable is a synonym for table_type. It was added in MySQL 4.1.2.
o sync_binlog If the value of this variable is positive, the MySQL server synchronizes its binary log to disk (using fdatasync()) after every sync_binlog writes to the binary log. Note that there is one write to the binary log per statement if autocommit is enabled, and one write per transaction otherwise. The default value is 0, which does no synchronizing to disk. A value of 1 is the safest choice, because in the event of a crash you lose at most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast). This variable was added in MySQL 4.1.3.
If the value of sync_binlog is 0 (the default), no extra flushing is done. The server relies on the operating system to flush the file contents occasionaly as for any other file.
o sync_frm If this variable is set to 1, when any non-temporary 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.
o system_time_zone 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. This variable was added in MySQL 4.1.3.
o table_cache 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 the section called 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 4.8, How MySQL Opens and Closes Tables.
o table_type 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 the section called \FBMYSQLD\FR COMMAND OPTIONS.
o thread_cache_size 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 doesn’t provide a notable performance improvement if you have a good thread implementation.) By examining the difference between the Connections and Threads_created status variables, you can see how efficient the thread cache is. For details, see the section called SERVER STATUS VARIABLES. This variable was added in MySQL 3.23.16.
o thread_concurrency On Solaris, mysqld calls thr_setconcurrency() with this 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.
o thread_stack 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 1.4, The MySQL Benchmark Suite. The default is 64KB before MySQL 4.0.10 and 192KB thereafter.
o time_format This variable is not implemented.
o time_zone The current time zone. This variable is used to initialize the tome 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 9.8, MySQL Server Time Zone Support. This variable was added in MySQL 4.1.3.
o timezone 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 4.6, Time Zone Problems.
o tmp_table_size If an in-memory temporary table exceeds this size, MySQL automatically converts it to an on-disk MyISAM table. Increase the value of tmp_table_size if you do many advanced GROUP BY queries and you have lots of memory.
o tmpdir 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