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. > > 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?
David J.
