This is slightly besides the topic, but since it can be of much help it's worth to mention in case someone learns something: primary keys are indexes per their

This is slightly besides the topic, but since it can be of much help it's worth to mention in case someone learns something: primary keys are indexes per their own nature. The KEY keyword is actually an alias for INDEX in pretty much all RDBMS. So, if you can't index an already large database for whatever reason, and happen to have a primary key column that could be used in a similar way, you're suddenly better off. It's also the fastest index available.