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•Running ANALYZE TABLE . 

•Running SHOW TABLE STATUS , SHOW INDEX , or querying the INFORMATION_SCHEMA.TABLES  or INFORMATION_SCHEMA.STATISTICS  tables with the innodb_stats_on_metadata  option enabled. 

The default setting for innodb_stats_on_metadata  is OFF . Enabling innodb_stats_on_metadata  may 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  tables. innodb_stats_on_metadata  is configured globally using a SET  statement. 

SET GLOBAL innodb_stats_on_metadata=ON 

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## Note 

innodb_stats_on_metadata  only applies when optimizer statistics are configured to be non-persistent (when innodb_stats_persistent  is disabled). 

•Starting a mysql  client with the --auto-rehash  option enabled, which is the default. The auto- rehash  option causes all InnoDB  tables 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- rehash  using the --disable-auto-rehash  option. The auto-rehash  feature enables automatic name completion of database, table, and column names for interactive users. 

•A table is first opened. 

• InnoDB  detects that 1 / 16 of table has been modified since the last time statistics were updated. 

## Configuring the Number of Sampled Pages 

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 8.2.1.23, “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. 

•Increasing the innodb_stats_transient_sample_pages  value 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. 

