When testing queries, always remember to use realistic data. A common source of problems with MySQL is the query optimizer’s handling of test data. It often does surprising things. If you don’t know what it’s doing and why (and it rarely tells you why), you may spend a lot of time tracking down a problem that really isn’t there. Or, worse yet, you may embarrass yourself asking about it on the MySQL mailing list, only to learn that you’ve created the problem all on your own.
In general, MySQL uses an index when it is reasonably confident that doing so is more efficient than not doing so.
