The problem is that you don’t really know if your mock accurately implements the behavior of the database. The only way you could verify the mock behaves correctly would be to run the mock and a real database instance through a set of tests to verify they both implement different isolation levels identically.

At that point—why bother with the mock at all? Cutting out the intermediary and running integration tests of your application against a real database will be faster.

You can’t use SQLite here either because all SQLite isolation levels are serializable anyway, which is _very_ different from how PostgreSQL works. Testing against SQLite could end up giving you a false sense of security that your code is safe against race conditions, whereas in reality it’s vulnerable when connected to a PostgreSQL server because of the difference in default isolation levels

At this level of testing detail the only real option is to test against a real database that matches what you’re running in production. Otherwise you’re just testing a mock.