Re: EXISTS by itself vs SELECT EXISTS much slower in query. - Mailing list pgsql-performance

Jimmy A <jimmypsql@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