MySQL/MariaDB – optimizer_switch

To control optimizer behaviour, we can enable/disable specific optimization via optimizer_switch system variable. The optimizer_switch variable can be changed at runtime, and has global and session values.

Execute the command below to see the current set of optimizer flags.`

SELECT @@optimizer_switch \G

To change the value of optimizer_switch, use the following syntax.

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

Syntax	Description
default	Reset all optimizations to their default values.
optimization_name=default	Set the specified optimization to its default value.
optimization_name=on	Enable the specified optimization.
optimization_name=off	Disable the specified optimization.

Cheers!