Controlling Switchable Optimizations

The optimizer_switch system variable enables control over optimizer behavior. Its value is a set of flags, each of which has a value ofon or off to indicate whether the corresponding optimizer behavior is enabled or disabled. This variable has global and session values and can be changed at runtime. The global default can be set at server startup.

To see the current set of optimizer flags, select the variable value:

mysql> SELECT @@optimizer_switch\G*************************** 1. row ***************************@@optimizer_switch: index_merge=on,index_merge_union=on,                    index_merge_sort_union=on,                    index_merge_intersection=on

To change the value of optimizer_switch, assign a value consisting of a comma-separated list of one or more commands:

SET [GLOBAL|SESSION] optimizer_switch='command[,command]...';

Each command value should have one of the forms shown in the following table.

Command SyntaxMeaningdefaultReset every optimization to its default valueopt_name=defaultSet the named optimization to its default valueopt_name=offDisable the named optimizationopt_name=onEnable the named optimization

The order of the commands in the value does not matter, although thedefault command is executed first if present. Setting anopt_name flag to default sets it to whichever of on or off is its default value. Specifying any given opt_name more than once in the value is not permitted and causes an error. Any errors in the value cause the assignment to fail with an error, leaving the value of optimizer_switch unchanged.

The following table lists the permissibleopt_name flag names.

Flag NameMeaningindex_mergeControls all Index Merge optimizationsindex_merge_intersectionControls the Index Merge Intersection Access optimizationindex_merge_sort_unionControls the Index Merge Sort-Union Access optimizationindex_merge_unionControls the Index Merge Union Access optimization

For information about Index Merge, seeSection 8.3.1.4, “Index Merge Optimization”.

When you assign a value to optimizer_switch, flags that are not mentioned keep their current values. This makes it possible to enable or disable specific optimizer behaviors in a single statement without affecting other behaviors. The statement does not depend on what other optimizer flags exist and what their values are. Suppose that all Index Merge optimizations are enabled:

mysql> SELECT @@optimizer_switch\G*************************** 1. row ***************************@@optimizer_switch: index_merge=on,index_merge_union=on,                    index_merge_sort_union=on,                    index_merge_intersection=on

If the server is using the Index Merge Union or Index Merge Sort-Union access methods for certain queries and you want to check whether the optimizer will perform better without them, set the variable value like this:

mysql> SET optimizer_switch='index_merge_union=off,index_merge_sort_union=off';mysql> SELECT @@optimizer_switch\G*************************** 1. row ***************************@@optimizer_switch: index_merge=on,index_merge_union=off,                    index_merge_sort_union=off,                    index_merge_intersection=on