Re: PG 13 release notes, first draft

On Wed, Jul 29, 2020 at 6:30 PM Bruce Momjian <bruce@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.

-- 
Peter Geoghegan