f4c00d138f
Since commit b448f1c8d, we've been able to remove left joins (that are otherwise removable) even when they are underneath other left joins, a case that was previously prevented by a delay_upper_joins check. This is a clear improvement, but it has a surprising side-effect: it's now possible that there are EquivalenceClasses whose relid sets mention the removed baserel and/or outer join. If we fail to clean those up, we may drop essential join quals due to not having any join level that appears to satisfy their relid sets. (It's not quite 100% clear that this was impossible before. But the lack of complaints since we added join removal a dozen years ago strongly suggests that it was impossible.) Richard Guo and Tom Lane, per bug #17976 from Zuming Jiang Discussion: https://postgr.es/m/17976-4b638b525e9a983b@postgresql.org
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