Re: Parallel Aggregates for string_agg and array_agg

On Tue, Aug 2, 2022 at 4:46 PM David Rowley <dgrowleyml@gmail.com> wrote:

> On Wed, 20 Jun 2018 at 19:08, David Rowley <david.rowley@2ndquadrant.com>
> wrote:
> > I'll submit it again when there more consensus that we want this.
>
> Waking up this old thread again. If you don't have a copy, the entire
> thread is in [1].
>
> The remaining item that seemed to cause this patch to be rejected was
> raised in [2]. The summary of that was that it might not be a good
> idea to allow parallel aggregation of string_agg() and array_agg() as
> there might be some people who rely on the current ordering they get
> without having an ORDER BY clause in the aggregate function call.  Tom
> mentioned in [3] that users might not want to add an ORDER BY to their
> aggregate function because the performance of it is terrible.  That
> was true up until 1349d2790 [4], where I changed how ORDER BY /
> DISTINCT aggregation worked to allow the planner to provide pre-sorted
> input rather than always having nodeAgg.c do the sorting.  I think
> this removes quite a lot of the argument against the patch, but not
> all of it.  So here goes testing the water on seeing if any opinions
> have changed over the past few years?
>
> A rebased patch is attached.
>
> David
>
> [1]
> https://www.postgresql.org/message-id/flat/CAKJS1f98yPkRMsE0JnDh72%3DAQEUuE3atiCJtPVCtjhFwzCRJHQ%40mail.gmail.com#8bbce15b9279d2da2da99071f732a99d
> [2] https://www.postgresql.org/message-id/6538.1522096067@sss.pgh.pa.us
> [3] https://www.postgresql.org/message-id/18594.1522099194@sss.pgh.pa.us
> [4]
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=1349d2790bf48a4de072931c722f39337e72055e

Hi,
For array_agg_combine():

+       if (state1->alen < reqsize)
+       {
+           /* Use a power of 2 size rather than allocating just reqsize */
+           state1->alen = pg_nextpower2_32(reqsize);
...
+       state1->nelems = reqsize;

I wonder why pg_nextpower2_32(reqsize) is used in the if block. It seems
reqsize should suffice.

Cheers