I see, I never knew that. Indeed there is a hashed subplan for the EXISTS by itself. So that explains it.
Thanks Tom. On Mon, Nov 8, 2021 at 12:35 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > Jimmy A <jimmyp...@gmail.com> writes: > > I have two equivalent queries, one with an EXISTS clause by itself and > one > > wrapped in a (SELECT EXISTS) and the "naked" exists is much slower. > > I would expect both to be the same speed / have same execution plan. > > That is a dangerous assumption. In general, wrapping (SELECT ...) around > something has a significant performance impact, because it pushes Postgres > to try to decouple the sub-select's execution from the outer query. > As an example, > > postgres=# select x, random() from generate_series(1,3) x; > x | random > ---+--------------------- > 1 | 0.08595356832524814 > 2 | 0.6444265043474005 > 3 | 0.6878852071694332 > (3 rows) > > postgres=# select x, (select random()) from generate_series(1,3) x; > x | random > ---+-------------------- > 1 | 0.7028987801136708 > 2 | 0.7028987801136708 > 3 | 0.7028987801136708 > (3 rows) > > That's not a bug: it's expected that the second query will evaluate > random() only once. > > In the case at hand, I suspect you're getting a "hashed subplan" > in one query and not the other. The depesz.com display doesn't > really show that, but EXPLAIN VERBOSE would. > > regards, tom lane >
