From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
---|---|
To: | Bruce Momjian <bruce(at)momjian(dot)us> |
Cc: | 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 02:00:43 |
Message-ID: | CAH2-Wznuah68LrWQFM2k9a7dZFzF_cgxfPi6sZLq8j4u_d2KPQ@mail.gmail.com |
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Lists: | pgsql-hackers |
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.
--
Peter Geoghegan
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