[vidarh]

MySQL has or had a way of forcing index use, and while it was very occasionally a life-safer, it was much less useful than you might think, as often when a DB engine falls back to sequential scan, it's for a reason (e.g. the query planner might have found that the indexes don't cover the columns you need, and the number of indexed lookups into the table that is needed are costly enough that a sequential scan might end up being faster, for example).

It was useful occasionally "back in the day" when MySQL's query planner was really bad, but today it will mostly appear useful if there's something subtly wrong with your config. I don't use MySQL much any more, but on Postgres one typical mistake might be that the costs configured for the query planner doesn't match your hardware (e.g. if your seek cost is configured to be high enough relative to sequential reads, sequential scans starts to look really good even when you need only a small portion of the data; principle will be the same on MySQL but I don't know if you have the same control over the query planner costs or not).