Robert Haas <robertmh...@gmail.com> writes: > On Tue, Jun 21, 2016 at 4:18 PM, Merlin Moncure <mmonc...@gmail.com> wrote: >> explain analyze select * from foo where false or exists (select 1 from >> bar where good and foo.id = bar.id); -- A >> explain analyze select * from foo where exists (select 1 from bar >> where good and foo.id = bar.id); -- B >> >> These queries are trivially verified as identical but give very different >> plans.
> Right. I suspect wouldn't be very hard to notice the special case of > FALSE OR (SOMETHING THAT MIGHT NOT BE FALSE) but I'm not sure that's > worth optimizing by itself. Constant-folding will get rid of the OR FALSE (as well as actually-useful variants of this example). The problem is that that doesn't happen till after we identify semijoins. So the second one gives you a semijoin plan and the first doesn't. This isn't especially easy to improve. Much of the value of doing constant-folding would disappear if we ran it before subquery pullup + join simplification, because in non-stupidly-written queries those are what expose the expression simplification opportunities. We could run it twice but that seems certain to be a dead loser most of the time. > A more promising line of attack as it > seems to me is to let the planner transform back and forth between > this form for the query and the UNION form. Maybe, but neither UNION nor UNION ALL would duplicate the semantics of OR, so there's some handwaving here that I missed. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers