On Wed, Jul 29, 2020 at 08:43:24PM -0700, David G. Johnston wrote:
> On Wednesday, July 29, 2020, Peter Geoghegan <p...@bowt.ie> wrote:
>
> On Wed, Jul 29, 2020 at 6:30 PM Bruce Momjian <br...@momjian.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 <br...@momjian.us> https://momjian.us
EnterpriseDB https://enterprisedb.com
The usefulness of a cup is in its emptiness, Bruce Lee
diff --git a/doc/src/sgml/release-13.sgml b/doc/src/sgml/release-13.sgml
new file mode 100644
index 18e6497..0b1051b
*** a/doc/src/sgml/release-13.sgml
--- b/doc/src/sgml/release-13.sgml
*************** Author: Jeff Davis <jda...@postgresql.or
*** 649,655 ****
<para>
Previously, hash aggregation was avoided if it was expected to use
! more than <xref linkend="guc-work-mem"/> memory.
</para>
</listitem>
--- 649,658 ----
<para>
Previously, hash aggregation was avoided if it was expected to use
! more than <xref linkend="guc-work-mem"/> memory. To reduce the
! likelihood of using disk storage for hash aggregation and attain
! behavior similar to previous Postgres releases, increase <xref
! linkend="guc-hash-mem-multiplier"/>.
</para>
</listitem>
