Re: Default setting for enable_hashagg_disk (hash_mem)

On Thu, Jul 2, 2020 at 10:58:34PM -0400, Bruce Momjian wrote:
> On Thu, Jul 2, 2020 at 09:46:49PM -0500, Justin Pryzby wrote:
> > On Thu, Jul 02, 2020 at 07:05:48PM -0700, Peter Geoghegan wrote:
> > > anything else). I still think that the new GUC should work as a
> > > multiplier of work_mem, or something else along those lines, though
> > > for now it's just an independent work_mem used for hashing. I bring it
> > > up again because I'm concerned about users that upgrade to Postgres 13
> > > incautiously, and find that hashing uses *less* memory than before.
> > > Many users probably get away with setting work_mem quite high across
> > > the board. At the very least, hash_mem should be ignored when it's set
> > > to below work_mem (which isn't what the patch does).
> >
> > I feel it should same as work_mem, as it's written, and not a multiplier.
> >
> > And actually I don't think a lower value should be ignored: "mechanism not
> > policy". Do we refuse atypical values of maintenance_work_mem < work_mem ?
> >
> > I assumed that hash_mem would default to -1, which would mean "fall back to
> > work_mem". We'd then advise users to increase it if they have an issue in v13
> > with performance of hashes spilled to disk. (And maybe in other cases, too.)
>
> Uh, with this patch, don't we really have sort_mem and hash_mem, but
> hash_mem default to sort_mem, or something like that. If hash_mem is a
> multiplier, it would make more sense to keep the work_mem name.

Also, I feel this is all out of scope for PG 13, frankly. I think our
only option is to revert the hash spill entirely, and return to PG 13
behavior, if people are too worried about hash performance regression.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee