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If the column being modified is referenced by a foreign key constraint of another table, ALTER TABLE would open the other table (to re-parse the constraint's definition) without having first obtained a lock on it. This was evidently intentional, but that doesn't mean it's really safe. It's especially not safe in 9.3, which pre-dates use of MVCC scans for catalog reads, but even in current releases it doesn't seem like a good idea. We know we'll need AccessExclusiveLock shortly to drop the obsoleted constraint, so just get that a little sooner to close the hole. Per testing with a patch that complains if we open a relation without holding any lock on it. I don't plan to back-patch that patch, but we should close the holes it identifies in all supported branches. Discussion: https://postgr.es/m/2038.1538335244@sss.pgh.pa.us