I agree that it's an opt-in behavior. My personal opinion is that having no way of restoring the old behavior is the biggest pain point in this upgrade more than anything. I think you need to include some information about migrating this situation for users here: https://github.com/hibernate/hibernate-orm/blob/5.0/migration-guide.adoc, including hopefully a list of databases and their old/new behavior for catalog/schema. I'm probably going to end up working around this by implementing a custom dialect that overrides the catalog/schema setting.

FWIW, when I read through the formal definition of catalog/schema, I would consider the mysql implementation to be catalog == server you connect to, schema == database that contains your tables/procedures, etc. I can't find catalog anywhere in their docs so it really puzzles me why their DatabaseMetaData object returns attributes indicating it has catalog support.