Re: PG 13 release notes, first draft

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>
Cc: Peter Geoghegan <pg(at)bowt(dot)ie>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: PG 13 release notes, first draft
Date: 2020-07-30 17:32:48
Message-ID: 20200730173248.GA16709@momjian.us
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On Wed, Jul 29, 2020 at 08:43:24PM -0700, David G. Johnston wrote:
> On Wednesday, July 29, 2020, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Wed, Jul 29, 2020 at 6:30 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > > There should be a note about this in the Postgres 13 release notes,
> > > for the usual reasons. More importantly, the "Allow hash aggregation
> > > to use disk storage for large aggregation result sets" feature should
> > > reference the new GUC directly. Users should be advised that the GUC
> > > may be useful in cases where they upgrade and experience a performance
> > > regression linked to slower hash aggregation. Just including a
> > > documentation link for the GUC would be very helpful.
> >
> > I came up with the attached patch.
>
> I was thinking something along like the following (after the existing
> sentence about avoiding hash aggs in the planner):
>
> If you find that hash aggregation is slower than in previous major
> releases of PostgreSQL, it may be useful to increase the value of
> hash_mem_multiplier. This allows hash aggregation to use more memory
> without affecting competing query operations that are generally less
> likely to put any additional memory to good use.

I came up with a more verbose documentation suggestion, attached.

> How about adding wording for GROUP BY as well to cater to users who are more
> comfortable thinking in terms of SQL statements instead of execution plans?

Uh, it is unclear exactly what SQL generates what node types, so I want
to avoid this.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee

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