Jimmy A <jimmypsql@xxxxxxxxx> 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
