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
>

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