Re: PG 13 release notes, first draft

On Wed, Jul 29, 2020 at 07:00:43PM -0700, Peter Geoghegan 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.

Well, that seems to be repeating what is already in the docs for
hash_mem_multiplier, which I try to avoid. One other direction is to
put something in the incompatibilities section. Does that make sense?

--
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