optimizer_adjust_secondary_key_costs

optimizer_adjust_secondary_key_costs

• Description: Gives the user the ability to affect how the costs for secondary keys using ref are calculated in the few cases when MariaDB 10.6 up to MariaDB 10.11 makes a sub-optimal choice when optimizing ref access, either for key lookups or GROUP BY. ref, as used by EXPLAIN, means that the optimizer is using key-lookup on one value to find the matching rows from a table. Unused from MariaDB 11.0. In MariaDB 10.6.18 the variable was changed from a number to a set of strings and disable_forced_index_in_group_by (value 4) was added.

• Scope: Global, Session

• Dynamic: Yes

• Data Type: set

• Default Value: fix_reuse_range_for_ref, fix_card_multiplier

• Range: 0 to 63 or any combination of adjust_secondary_key_cost, disable_max_seek or disable_forced_index_in_group_by, fix_innodb_cardinality,fix_reuse_range_for_ref, fix_card_multiplier

• Introduced: MariaDB 10.6.17, MariaDB 10.11.7

MariaDB [securedb]> select @@version;
+-----------------------------------+
| @@version             |
+-----------------------------------+
| 10.6.20-16-MariaDB-enterprise-log |
+-----------------------------------+
1 row in set (0.001 sec)

MariaDB [securedb]> select @@optimizer_adjust_secondary_key_costs;
+---------------------------------------------+
| @@optimizer_adjust_secondary_key_costs   |
+---------------------------------------------+
| fix_reuse_range_for_ref,fix_card_multiplier |
+---------------------------------------------+

MariaDB starting with 11.0

optimizer_adjust_secondary_key_costs will be obsolete starting from MariaDB 11.0 as the new optimizer in 11.0 does not have max_seek optimization and is already using cost based choices for index usage with GROUP BY.

The value for optimizer_adjust_secondary_key_costs is one of more of the following:

Value	Version added	Old behavior	Change when variable is used
adjust_secondary_key_cost	10.6.17	Limit ref costs by max_seeks	The secondary key costs for ref are updated to be at least five times the clustered primary key costs if a clustered primary key exists
disable_max_seek	10.6.17	ref cost on secondary keys is limited to max_seek = min('number of expected rows'/ 10, scan_time*3)	Disable 'max_seek' optimization and do a slight adjustment of filter cost
disable_forced_index_in_group_by	10.6.18	Use a rule-based choice when deciding to use an index to resolve GROUP BY	The choice is now cost based
fix_innodb_cardinality	10.6.19	By default InnoDB doubles the cardinality for indexes in an effort to force index usage over table scans. This can cause the optimizer to create sub-optimal plans for ranges or index entries that cover a big part of the table.	Using this option removes the doubling of cardinality in InnoDB. fix_innodb_cardinality is recommended to be used only as a server startup option, as it is enabled for a table at first usage. See MDEV-34664 for details.
fix_reuse_range_for_ref	10.6.20	Number of estimated rows for 'ref' did not always match costs from range optimizer	Use cost from range optimizer for 'ref' if all used key parts are constants. The old code did not always do this
fix_card_multiplier	10.6.20	Index selectivity can be bigger than 1.0 if index statistics is not up to date. Not on by default.	Ensure that the calculated index selectivity is never bigger than 1.0. Having index selectivity bigger than 1.0 causes MariaDB to believe that there is more rows in the table than in reality, which can cause wrong plans. This option is on by default.

One can set all options with:

SET @@optimizer_adjust_secondary_key_costs='all';

Explanations of the old behavior in MariaDB 10.x

The reason for the max_seek optimization was originally to ensure that MariaDB would use a key instead of a table scan. This works well for a lot of queries, but can cause problems when a table scan is a better choice, such as when one would have to scan more than 1/4 of the rows in the table (in which case a table scan is better).

See Also

• The optimizer_switch system variable.

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