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. -- Peter Geoghegan
