Horton

Re: Missing docs for new enable_group_by_reordering GUC

Pavel Borisov <pashkin.elfe@gmail.com>

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From: Pavel Borisov <pashkin.elfe@gmail.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Andrei Lepikhov <lepihov@gmail.com>, Bruce Momjian <bruce@momjian.us>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2024-06-18T12:45:25Z
Lists: pgsql-hackers

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  1. Add doc entry for the new GUC paramenter enable_group_by_reordering

  2. Explore alternative orderings of group-by pathkeys during optimization.

Hi, Alexander!

On Tue, 18 Jun 2024 at 16:13, Alexander Korotkov <aekorotkov@gmail.com>
wrote:

> On Tue, Jun 18, 2024 at 9:14 AM Andrei Lepikhov <lepihov@gmail.com> wrote:
> > On 6/18/24 09:32, Bruce Momjian wrote:
> > > This commit added enable_group_by_reordering:
> > >
> > >       commit 0452b461bc4
> > >       Author: Alexander Korotkov <akorotkov@postgresql.org>
> > >       Date:   Sun Jan 21 22:21:36 2024 +0200
> > > It mentions it was added as a GUC to postgresql.conf, but I see no SGML
> > > docs for this new GUC value.  Would someone please add docs for this?
> > > Thanks.
> > It is my mistake, sorry for that. See the patch in attachment.
>
> Bruce, thank for noticing.  Andrei, thank you for providing a fix.
> Please, check the revised patch.
>
I briefly looked into this docs patch. Planner gucs are arranged
alphabetically, so enable_group_by_reordering is better to come after
enable-gathermerge not before.

+  Enables or disables the reordering of keys in a
+        <literal>GROUP BY</literal> clause to match the ordering keys of a
+        child node of the plan, such as an index scan. When turned off,
keys
+        in a <literal>GROUP BY</literal> clause are only reordered to match
+        the <literal>ORDER BY</literal> clause, if any. The default is
+        <literal>on</literal>.
I'd also suggest the same style as already exists
for enable_presorted_aggregate guc i.e:

Controls if the query planner will produce a plan which will provide
<literal>GROUP BY</literal> keys presorted in the order of keys of a child
node of the plan, such as an index scan. When disabled, the query planner
will produce a plan with <literal>GROUP BY</literal> keys only reordered to
match
the <literal>ORDER BY</literal> clause, if any. When enabled, the planner
will try to produce a more efficient plan. The default value is on.

Regards,
Pavel Borisov
Supabase