The regression test shows where two fully equal join clauses, applied on different join levels, differ by the only required_relids. It covers the problem and can be helpful by itself. IMO, one line in this test is redundant (see attachment).
Hmm, I don't think the insert statement in the test case is redundant. It's needed to verify that the query in the test case gives the correct result. Without the insert statement, the wrong plan would give the same result as the correct plan, i.e., an empty set in this case.
IMO, if we make some code changes and add a test case for that, we need to ensure the test can give a different (and correct of course) result than what came before.
