Make FOREIGN KEY constraint to use any suitable index

[hvlad]

Ability to automatically choose existing index to check FK constraint allows to lower number of indices and consume less disk and CPU resources.

[dyemanov]

#7258 ?

[hvlad]

This ticket a bit more wide than #7258 - it is about automatically choose existing index (no user interaction) vs explicitly set index to use by user.
But, details is not fixed yet, so it could be discussed if necessary.

[dyemanov]

Well, I read #7258 exactly as the engine should track other indices and use any suitable one automagically. But I don't mind if we also offer some manual way. Let's get back to it after the automatic solution is implemented.