Three years ago MySQL has got @@optimizer_switch variable. It was introduced in MySQL 5.1.34, that is, we needed it so much that we’ve added it into the stable release.

In a nutshell,@@optimizer_switch held a comma-separated list of optimizer controls:

mysq> > select @@optimizer_switch;
+------------------------------------------------------------------------------------------+
| @@optimizer_switch                                                                       |
+------------------------------------------------------------------------------------------+
| index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on|
+------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

One could set all settings at once:

mysql> set …