David Rowley <david.row...@2ndquadrant.com> writes: > I can make this change, but before I do I just want to point that I > don't think what you've said here is entirely accurate.
> Let's assume unique joins are very common place, and join removals are > not so common. If a query has 5 left joins, and only one of which can > be removed, then the new code will most likely perform 5 unique join > checks, whereas the old code would perform 9, as those unique checks > are performed again once the 1 relation is removed for the remaining > 4. I'm not following. If the join removal code had reached the stage of making a uniqueness check, and that check had succeeded, the join would be gone and there would be no repeat check later. If it didn't reach the stage of making a uniqueness check, then again there's no duplication. There will be some advantage in making a negative cache entry if join removal performs a uniqueness check that fails, but I don't really see why that's hard. It does not seem like removal of a relation could cause another rel to become unique that wasn't before, so keeping negative cache entries across join removals ought to be safe. regards, tom lane -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers