The Ultimate Guide to High Availability with MariaDB

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: ""
  • Range: 0 to 7 or any combination of adjust_secondary_key_cost, disable_max_seek or disable_forced_index_in_group_by.
  • Introduced: MariaDB 10.6.17, MariaDB 10.11.7, MariaDB 11.0.5, MariaDB 11.1.4, MariaDB 11.2.3


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:

ValueVersion addedDefault behaviorChange when variable is used
adjust_secondary_key_cost10.6.17Limit ref costs by max_seeksThe 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_seek10.6.17ref 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_by10.6.18Use a rule-based choice when deciding to use an index to resolve GROUP BYThe choice is now cost based
fix_innodb_cardinality10.6.19By 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.

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).

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