Good catch! We started adding support for IF EXISTS in https://hibernate.atlassian.net/browse/HHH-10605, but it looks like we need to extend it further.

First, we need to have this in PostgreSQL, H2, and also SQL Server 2016.

Now, I checked your PR, but I'd like to do it like we started with Dialect.supportsIfExistsAfterConstraintName, so we need something like {{Dialect.supportsIfExistsAfterAlterOrDropTable}.

Now, in the Jira issue description, you said that:

Implementing this would get rid of errors when dropping constraints on tables that might not exist

I wonder why the Dialect.supportsIfExistsAfterConstraintName did not handle this properly.