Benny Kramek <benny@xxxxxxxxxxx> writes: > I expect the query plan to be identical for both of the below queries (and I > expect the performance to also be identical). [ shrug... ] Your expectation is mistaken. There is no code in Postgres to eliminate useless self-joins. People have been fooling around with a patch to do so [1], but I'm unsure whether it'll ever get committed, or whether we even want the feature. It seems not unlikely that the time wasted trying to identify useless self-joins (in queries where the optimization doesn't actually apply) would outweigh the win when it does apply. So there's a limit to how much effort the server should spend trying to clean up after poorly-written queries. regards, tom lane [1] https://www.postgresql.org/message-id/flat/64486b0b-0404-e39e-322d-0801154901f3@xxxxxxxxxxxxxx
