MySQL indexes - how many are enough?

Question

I\'m trying to fine-tune my MySQL server so I check my settings, analyzing slow-query log, and simplify my queries if possible.

Sometimes it is enough if I am indexi

Answer 1

As a general rule, you should have indexes on all primary keys (you don't have a choice in that), all foreign keys, and any other fields you commonly use to fetch rows.

For example, if I commonly look up users by username, I would have that indexed, even if user ID was the primary key.