Re: EXISTS by itself vs SELECT EXISTS much slower in query.

Jimmy A <jimmypsql(at)gmail(dot)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