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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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 <tgl@xxxxxxxxxxxxx> wrote:
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

[Postgresql General]     [Postgresql PHP]     [PHP Users]     [PHP Home]     [PHP on Windows]     [Kernel Newbies]     [PHP Classes]     [PHP Books]     [PHP Databases]     [Yosemite]

  Powered by Linux