22.214.171.124 Configuring Non-Persistent Optimizer Statistics Parameters
This section describes how to configure non-persistent optimizer statistics. Optimizer statistics are not persisted to disk when
innodb_stats_persistent=OFF or when individual tables are created or altered with
STATS_PERSISTENT=0. Instead, statistics are stored in memory, and are lost when the server is shut down. Statistics are also updated periodically by certain operations and under certain conditions.
As of MySQL 5.6.6, optimizer statistics are persisted to disk by default, enabled by the
innodb_stats_persistent configuration option. For information about persistent optimizer statistics, see Section 126.96.36.199, “Configuring Persistent Optimizer Statistics Parameters”.
Non-persistent optimizer statistics are updated when:
The default setting for
innodb_stats_on_metadatawas changed to
OFFwhen persistent optimizer statistics were enabled by default in MySQL 5.6.6. Enabling
innodb_stats_on_metadatamay reduce access speed for schemas that have a large number of tables or indexes, and reduce stability of execution plans for queries that involve
innodb_stats_on_metadatais configured globally using a
SET GLOBAL innodb_stats_on_metadata=ON
Starting a mysql client with the
--auto-rehashoption enabled, which is the default. The
auto-rehashoption causes all
InnoDBtables to be opened, and the open table operations cause statistics to be recalculated.
To improve the start up time of the mysql client and to updating statistics, you can turn off
auto-rehashfeature enables automatic name completion of database, table, and column names for interactive users.
A table is first opened.
InnoDBdetects that 1 / 16 of table has been modified since the last time statistics were updated.
The MySQL query optimizer uses estimated statistics about key distributions to choose the indexes for an execution plan, based on the relative selectivity of the index. When
InnoDB updates optimizer statistics, it samples random pages from each index on a table to estimate the cardinality of the index. (This technique is known as random dives.)
To give you control over the quality of the statistics estimate (and thus better information for the query optimizer), you can change the number of sampled pages using the parameter
innodb_stats_transient_sample_pages. The default number of sampled pages is 8, which could be insufficient to produce an accurate estimate, leading to poor index choices by the query optimizer. This technique is especially important for large tables and tables used in joins. Unnecessary full table scans for such tables can be a substantial performance issue. See Section 188.8.131.52, “Avoiding Full Table Scans” for tips on tuning such queries.
innodb_stats_transient_sample_pages is a global parameter that can be set at runtime.
The value of
innodb_stats_transient_sample_pages affects the index sampling for all
InnoDB tables and indexes when
innodb_stats_persistent=0. Be aware of the following potentially significant impacts when you change the index sample size:
Small values like 1 or 2 can result in inaccurate estimates of cardinality.
innodb_stats_transient_sample_pagesvalue might require more disk reads. Values much larger than 8 (say, 100), can cause a significant slowdown in the time it takes to open a table or execute
SHOW TABLE STATUS.
The optimizer might choose very different query plans based on different estimates of index selectivity.
Whatever value of
innodb_stats_transient_sample_pages works best for a system, set the option and leave it at that value. Choose a value that results in reasonably accurate estimates for all tables in your database without requiring excessive I/O. Because the statistics are automatically recalculated at various times other than on execution of
ANALYZE TABLE, it does not make sense to increase the index sample size, run
ANALYZE TABLE, then decrease sample size again.
Smaller tables generally require fewer index samples than larger tables. If your database has many large tables, consider using a higher value for
innodb_stats_transient_sample_pages than if you have mostly smaller tables.