Re: Default setting for enable_hashagg_disk (hash_mem)

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.

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

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