For additional anecdotes, I've had to hint MySQL's planner twice, both times to exclude an index. It'd switch because the cardinality of one index was much bet

For additional anecdotes, I've had to hint MySQL's planner twice, both times to exclude an index. It'd switch because the cardinality of one index was much better, but there were enough pathological cases (and they were the most-often used) that the planner's choices were proving fatal. It's not too abnormal. Sometimes you know the structure of your data better than the database's internal summary does, so you just give it a nudge in the right direction.