USE/FORCE/IGNORE INDEX syntax, or index hints, are nice shortcuts to make sure that MySQL will (or will not) use a certain index. But it comes with some drawbacks:
USE/FORCE INDEX will not allow to use an index not mentioned in the list
This could be by design, though in the case of USE INDEX it sounds weird to me. Why? Because if none of the indexes mentioned in the list is usable, a full table scan will happen.
Why is this a problem? Because in the real world queries are generated dynamic and evolve over time. Today’s optimisations could be tomorrow’s wrong hints. I had a case of a wrong USE INDEX preventing the use of the primary key.
Produces an error if the index doesn’t exist
Again, this could be by design, but in the case of IGNORE INDEX this seems to me not ideal. A warning would be much better. Even better, I’d like to have this behaviour governed by a variable.
Why is this a problem? Because indexes can be deleted. Maybe it’s because queries change, maybe it’s because they were wrong from the start (possibly not even created by a DBA). But then, dropping an index can generate errors for existing applications.
Unfortunately, IMHO, documenting the usage of such hints is too difficult.
EDIT: On my professional website, I’ve published The prerils of ALTER TABLE in MySQL/MariaDB. It includes the problem discussed here of course, but it is a more comprehensive overview of the problems you may encounter during a migration.