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If the inner relation can be proven unique, that is it can have no more than one matching row for any row of the outer query, then we might as well implement the semijoin as a plain inner join, allowing substantially more freedom to the planner. This is a form of outer join strength reduction, but it can't be implemented in reduce_outer_joins() because we don't have enough info about the individual relations at that stage. Instead do it much like remove_useless_joins(): once we've built base relations, we can make another pass over the SpecialJoinInfo list and get rid of any entries representing reducible semijoins. This is essentially a followon to the inner-unique patch (commit 9c7f5229a) and makes use of the proof machinery that that patch created. We need only minor refactoring of innerrel_is_unique's API to support this usage. Per performance complaint from Teodor Sigaev. Discussion: https://postgr.es/m/f994fc98-389f-4a46-d1bc-c42e05cb43ed@sigaev.ru
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