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

To see the current values used by a running server, use the SHOW VARIABLES statement.

Section 5.2.4, “Using System Variables”, discusses the syntax for setting and displaying system variable values.

Section 5.2.4.2, “Dynamic System Variables”, lists the variables that can be set at runtime.

Information on tuning sytem variables can be found in Section 7.5.2, “Tuning Server Parameters”.

Section 14.2.5, “InnoDB Startup Options and System Variables”, lists InnoDB system variables.

ansi_mode

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.

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.

basedir

The MySQL installation base directory. This variable can be set with the --basedir option.

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.

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.

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.

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.

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.

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.

bdb_tmpdir

The BDB temporary file directory. This variable was added in MySQL 3.23.14.

bdb_version

See the description for version_bdb.

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 5.11.4, “The Binary Log”.

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. Before MySQL 4.0.3. this variable was named myisam_bulk_insert_tree_size.

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.

character_set_client

The character set for statements that arrive from the client. This variable was added in MySQL 4.1.1.

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.

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.

character_set_results

The character set used for returning query results to the client. This variable was added in MySQL 4.1.1.

character_set_server

The server default character set. This variable was added in MySQL 4.1.1.

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.

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.)

character_sets_dir

The directory where character sets are installed. This variable was added in MySQL 4.1.2.

collation_connection

The collation of the connection character set. This variable was added in MySQL 4.1.1.

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.

collation_server

The server default collation. This variable was added in MySQL 4.1.1.

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 of the data file. 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 7.3.3, “Concurrent Inserts”.

connect_timeout

The number of seconds that the mysqld server waits for a connect packet before responding with Bad handshake.

convert_character_set

The current character set mapping that was set by SET CHARACTER SET. This variable was removed in MySQL 4.1.

datadir

The MySQL data directory. This variable can be set with the --datadir option.

date_format

This variable is not implemented.

datetime_format

This variable is not implemented.

default_week_format

The default mode value to use for the WEEK() function. See Section 12.6, “Date and Time Functions”. This variable is available as of MySQL 4.0.14.

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.

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 5.2.2, “Command Options”, and Section 14.1.1, “MyISAM Startup Options”.

Note that 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.

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.

delayed_insert_timeout

How many seconds an INSERT DELAYED handler thread should wait for INSERT statements before terminating.

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.

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.

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 A.1.4.2, “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.

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.

ft_boolean_syntax

The list of operators supported by boolean full-text searches performed using IN BOOLEAN MODE. See Section 12.8.1, “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:

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.

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.

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.

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.

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.

have_archive

YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.3.

have_bdb

YES if mysqld supports BDB tables. DISABLED if --skip-bdb is used. This variable was added in MySQL 3.23.30.

have_blackhole_engine

YES if mysqld supports BLACKHOLE tables, NO if not. This variable was added in MySQL 4.1.11.

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.

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.

have_csv

YES if mysqld supports ARCHIVE tables, NO if not. This variable was added in MySQL 4.1.4.

have_example_engine

YES if mysqld supports EXAMPLE tables, NO if not. This variable was added in MySQL 4.1.4.

have_geometry

YES if the server supports spatial data types, NO if not. This variable was added in MySQL 4.1.3.

have_innodb

YES if mysqld supports InnoDB tables. DISABLED if --skip-innodb is used. This variable was added in MySQL 3.23.37.

have_isam

YES if mysqld supports ISAM tables. DISABLED if --skip-isam is used. This variable was added in MySQL 3.23.30.

have_ndbcluster

YES if mysqld supports NDB Cluster tables. DISABLED if --skip-ndbcluster is used. This variable was added in MySQL 4.1.2.

have_openssl

YES if mysqld supports SSL (encryption) connections, NO if not. This variable was added in MySQL 3.23.43.

have_query_cache

YES if mysqld supports the query cache, NO if not. This variable was added in MySQL 4.0.2.

have_raid

YES if mysqld supports the RAID option, NO if not. This variable was added in MySQL 3.23.30.

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.

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.

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.

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.

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.

innodb_xxx

InnoDB system variables are listed in Section 14.2.5, “InnoDB Startup Options and System Variables”.

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.

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.

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 7.2.13, “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 13.5.4, “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_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 7.4.6, “The MyISAM Key Cache”.

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 7.4.6, “The MyISAM Key Cache”.

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 7.4.6, “The MyISAM Key Cache”.

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 7.4.6, “The MyISAM Key Cache”.

language

The language used for error messages.

large_file_support

Whether mysqld was compiled with options for large file support. This variable was added in MySQL 3.23.28.

lc_time_names

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 5.10.9, “MySQL Server Locale Support”. This variable was added in MySQL 4.1.21.

license

The type of license the server has. This variable was added in MySQL 4.0.19.

local_infile

Whether LOCAL is supported for LOAD DATA INFILE statements. See Section 5.6.4, “Security Issues with LOAD DATA LOCAL”. This variable was added in MySQL 4.0.3.

locked_in_memory

Whether mysqld was locked in memory with --memlock. This variable was added in MySQL 3.23.25.

log

Whether logging of all statements to the general query log is enabled. See Section 5.11.2, “The General Query Log”.

log_bin

Whether the binary log is enabled. This variable was added in MySQL 3.23.14. See Section 5.11.4, “The Binary Log”.

log_error

The location of the error log. This variable was added in MySQL 4.0.10.

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 6.8, “Replication Startup Options”.

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 5.11.5, “The Slow Query Log”.

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 5.11.3, “The Update Log”.

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.

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 5.11.5, “The Slow Query Log”.

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. Before MySQL 3.22.5, this variable was named sql_low_priority_updates.

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.

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 9.2.2, “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.

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.

max_binlog_cache_size

If a multiple-statement transaction requires more than this many bytes of memory, the server generates a Multi-statement transaction required more than 'max_binlog_cache_size' bytes of storage error. The minimum value is 4096, the maximum and default values are 4GB. This variable was added in MySQL 3.23.29.

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.

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.

max_connections

The number of simultaneous client connections allowed. By default, this is 100. See Section A.1.2.6, “Too many connections”, for more information.

MySQL Enterprise.  For notification that the maximum number of connections is getting dangerously high and for advice on setting the optimum value for max_connections subscribe to the MySQL Network Monitoring and Advisory Service. For more information, see http://www.mysql.com/products/enterprise/advisors.html.

Increasing this value increases the number of file descriptors that mysqld requires. See Section 7.4.8, “How MySQL Opens and Closes Tables”, for comments on file descriptor limits.

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.

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.

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.

MySQL Enterprise.  Subscribers to the MySQL Network Monitoring and Advisory Service receive recommendations for the optimum setting for max_heap_table_size. For more information see, http://www.mysql.com/products/enterprise/advisors.html.

max_insert_delayed_threads

This variable is a synonym for max_delayed_threads. It was added in MySQL 4.0.19.

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.

max_length_for_sort_data

The cutoff on the size of index values that determines which filesort algorithm to use. See Section 7.2.9, “ORDER BY Optimization”. This variable was added in MySQL 4.1.1

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 million. 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.

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 6.3, “Replication Implementation Details”.

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 13.5.4.11, “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.

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.

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.)

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.

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.

myisam_block_size

The block size to be used for MyISAM index pages.

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 A.1.2.11, “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. Note: The value is given in megabytes before 4.0.3 and in bytes thereafter.

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.

The default value is 2GB. If MyISAM index files exceed this size and disk space is available, increasing the value may help performance.

myisam_recover_options

The value of the --myisam-recover option. See Section 5.2.2, “Command Options”. This variable was added in MySQL 3.23.36.

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.

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.

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 7.4.7, “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.

named_pipe

On Windows, indicates whether the server supports connections over named pipes. This variable was added in MySQL 3.23.50.

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.

ndb_cache_check_time

The number of milliseconds 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 on every query.

The recommended maximum value for this variable is 1000, which means that the query cache is checked once per second. A larger value means the NDB query cache is less often checked and invalidated due to updates on a different mysqld. It is generally not desirable to set this to a value greater than 2000.

ndb_force_send

Forces sending of buffers to NDB immediately, without waiting for other threads. Defaults to ON.

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 queried directly. Default value: 32.

ndb_index_stat_enable

Use NDB index statistics in query optimization. Defaults to ON.

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 20^th query to the data nodes.

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.

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.

ndb_use_exact_count

Forces NDB to use a 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.

ndb_use_transactions

You can disable NDB transaction support by setting this variable's values to OFF (not recommended). The default is ON.

net_buffer_length

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.

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.

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.

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.

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.

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.

one_shot

This is not a variable, but it can be used when setting some variables. It is described in Section 13.5.3, “SET Syntax”.

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.

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.

port

The number of the port on which the server listens for TCP/IP connections. This variable can be set with the --port option.

preload_buffer_size

The size of the buffer that is allocated when preloading indexes. This variable was added in MySQL 4.1.1.

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. In MySQL 4.1.23, it was converted to the global Prepared_stmt_count status variable.

protocol_version

The version of the client/server protocol used by the MySQL server. This variable was added in MySQL 3.23.18.

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.

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.

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 5.13.3, “Query Cache Configuration”. This variable is present from MySQL 4.1.

query_cache_size

The amount of memory allocated for caching query results. The default value is 0, which disables the query cache. The allowable 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.

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.

This variable was added in MySQL 4.0.3.

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.

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.

range_alloc_block_size

The size of blocks that are allocated when doing range optimization. This variable was added in MySQL 4.0.16.

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.

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 7.5.4, “How MySQL Uses Memory”, for example.

Before MySQL 4.0.3, this variable was named record_buffer.

read_only

When this variable is set to ON, the server allows 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.

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.

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 7.5.4, “How MySQL Uses Memory”, for example.

Before MySQL 4.0.3, this variable was named record_rnd_buffer.

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.

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.

rpl_recovery_rank

This variable is unused.

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.

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.

shared_memory

(Windows only.) Whether the server allows shared-memory connections. This variable was added in MySQL 4.1.1.

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.

skip_external_locking

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.

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.

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.

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.

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.

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.

slave_skip_errors

The replication errors that the slave should skip (ignore). This variable was added in MySQL 3.23.47.

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.

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.

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).

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 A.1.4.4, “Where MySQL Stores Temporary Files”.

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 Section 5.2.6, “SQL Modes”.

sql_slave_skip_counter

The number of events from the master that a slave server should skip. See Section 13.6.2.6, “SET GLOBAL SQL_SLAVE_SKIP_COUNTER Syntax”. This variable was added in MySQL 3.23.33.

storage_engine

This variable is a synonym for table_type. It was added in MySQL 4.1.2.

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.

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.

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.

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 5.10.8, “MySQL Server Time Zone Support”.

system_time_zone was added in MySQL 4.1.3.

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 Section 5.2.5, “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 7.4.8, “How MySQL Opens and Closes Tables”.

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 Section 5.2.2, “Command Options”.

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 Section 5.2.5, “Status Variables”. This variable was added in MySQL 3.23.16.

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.

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 7.1.4, “The MySQL Benchmark Suite”. The default is 64KB before MySQL 4.0.10 and 192KB thereafter.

time_format

This variable is not implemented.

time_zone

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 5.10.8, “MySQL Server Time Zone Support”. This variable was added in MySQL 4.1.3.

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 A.1.4.6, “Time Zone Problems”.

tmp_table_size

The maximum size of in-memory temporary tables. (The actual limit is determined as the smaller of max_heap_table_size and tmp_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.

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 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 filesystem 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 won't use the general tmpdir value and you can set tmpdir to a non-permanent location.

This variable was added in MySQL 3.22.4.

transaction_alloc_block_size

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.

transaction_prealloc_size

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.

tx_isolation

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 13.4.6, “SET TRANSACTION Syntax”. If you set tx_isolation directly to an isolation level name that contains a space, the name should be enclosed within quotes, with the space replaced by a dash. For example:

SET tx_isolation = 'READ-COMMITTED';

version

The version number for the server.

version_bdb

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.

version_comment

The configure script has a --with-comment option that allows 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.

version_compile_machine

The type of machine or architecture on which MySQL was built. This variable was added in MySQL 4.1.1.

version_compile_os

The type of operating system on which MySQL was built. This variable was added in MySQL 4.0.19.

wait_timeout

The number of seconds the server waits for activity on a non-interactive connection before closing it. This timeout applies only to TCP/IP and Unix socket file connections, not to connections made via 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_t