Performance of querying across two mysql databases on the same server?

Question

Is there any performance hit from querying over two (or more) databases on the same MySQL server, compared to if those databases had been merged into one?

Background

Answer 1

No. In MySQL, "databases" are essentially just catalogues which have no effect on the way data are stored or queried.

Querying two tables in different databases is the same as querying two tables in the same db, from a query execution standpoint.