14.2.5.8. Controlling Optimizer Statistics Estimation

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. Certain operations cause InnoDB to sample random pages from each index on a table to estimate the cardinality of the index. (This technique is known as random dives.) These operations include the ANALYZE TABLE statement, the SHOW TABLE STATUS statement, and accessing the table for the first time after a restart.

To give you control over the quality of the statistics estimate (and thus better information for the query optimizer), you can now change the number of sampled pages using the parameter innodb_stats_sample_pages. Previously, the number of sampled pages was always 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 8.2.1.4, "How to Avoid Full Table Scans" for tips on tuning such queries.

You can set the global parameter innodb_stats_sample_pages, at runtime. The default value for this parameter is 8, preserving the same behavior as in past releases.

Note

The value of innodb_stats_sample_pages affects the index sampling for all InnoDB tables and indexes. There are the following potentially significant impacts when you change the index sample size:

Small values like 1 or 2 can result in very inaccurate estimates of cardinality.
Increasing the innodb_stats_sample_pages value might require more disk reads. Values much larger than 8 (say, 100), can cause a big 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.

To disable the cardinality estimation for metadata statements such as SHOW TABLE STATUS, execute the statement SET GLOBAL innodb_stats_on_metadata=OFF (or 0). The ability to set this option dynamically is also relatively new.

All InnoDB tables are opened, and the statistics are re-estimated for all associated indexes, when the mysql client starts if the auto-rehash setting is set on (the default). To improve the start up time of the mysql client, you can turn auto-rehash off. The auto-rehash feature enables automatic name completion of database, table, and column names for interactive users.

Whatever value of innodb_stats_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. The more accurate statistics calculated by ANALYZE running with a high value of innodb_stats_sample_pages can be wiped away later.

Although it is not possible to specify the sample size on a per-table basis, smaller tables generally require fewer index samples than larger tables do. If your database has many large tables, consider using a higher value for innodb_stats_sample_pages than if you have mostly smaller tables.