Re: [HACKERS] Partition-wise aggregation/grouping - Mailing list pgsql-hackers

Robert Haas <robertmhaas@gmail.com> wrote:

> On Wed, Apr 26, 2017 at 6:28 AM, Antonin Houska <ah@cybertec.at> wrote:
> > Attached is a diff that contains both patches merged. This is just to prove my
> > assumption, details to be elaborated later. The scripts attached produce the
> > following plan in my environment:
> >
> >                    QUERY PLAN
> > ------------------------------------------------
> >  Parallel Finalize HashAggregate
> >    Group Key: b_1.j
> >    ->  Append
> >          ->  Parallel Partial HashAggregate
> >                Group Key: b_1.j
> >                ->  Hash Join
> >                      Hash Cond: (b_1.j = c_1.k)
> >                      ->  Seq Scan on b_1
> >                      ->  Hash
> >                            ->  Seq Scan on c_1
> >          ->  Parallel Partial HashAggregate
> >                Group Key: b_2.j
> >                ->  Hash Join
> >                      Hash Cond: (b_2.j = c_2.k)
> >                      ->  Seq Scan on b_2
> >                      ->  Hash
> >                            ->  Seq Scan on c_2
>
> Well, I'm confused.  I see that there's a relationship between what
> Antonin is trying to do and what Jeevan is trying to do, but I can't
> figure out whether one is a subset of the other, whether they're both
> orthogonal, or something else.  This plan looks similar to what I
> would expect Jeevan's patch to produce,

The point is that the patch Jeevan wanted to work on is actually a subset of
[1] combined with [2].

> except i have no idea what "Parallel" would mean in a plan that contains no
> Gather node.

parallel_aware field was set mistakenly on the AggPath. Fixed patch is
attached below, producing this plan:

                   QUERY PLAN
------------------------------------------------
 Finalize HashAggregate
   Group Key: b_1.j
   ->  Append
         ->  Partial HashAggregate
               Group Key: b_1.j
               ->  Hash Join
                     Hash Cond: (b_1.j = c_1.k)
                     ->  Seq Scan on b_1
                     ->  Hash
                           ->  Seq Scan on c_1
         ->  Partial HashAggregate
               Group Key: b_2.j
               ->  Hash Join
                     Hash Cond: (b_2.j = c_2.k)
                     ->  Seq Scan on b_2
                     ->  Hash
                           ->  Seq Scan on c_2

[1] https://www.postgresql.org/message-id/9666.1491295317%40localhost

[2] https://commitfest.postgresql.org/14/994/

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at

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