Re: Default setting for enable_hashagg_disk (hash_mem)

On Thu, Jul 2, 2020 at 7:46 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:
> Thanks for putting it together, I agree that hash_mem seems to be an obvious
> "escape hatch" that generalizes existing GUCs and independently useful.

It is independently useful. It's a natural consequence of "being
honest" about work_mem and hashing.

> 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 see your point, but AFAIK maintenance_work_mem was not retrofit like
this. It seems different. (Unless we add the -1 behavior, perhaps)

> 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.)

Yeah, this kind of -1 behavior could make sense.

> I read the argument that hash tables are a better use of RAM than sort.
> However it seems like setting the default to greater than work_mem is a
> separate change than providing the mechanism allowing user to do so. I guess
> the change in default is intended to mitigate the worst possible behavior
> change someone might experience in v13 hashing, and might be expected to
> improve "out of the box" performance. I'm not opposed to it, but it's not an
> essential part of the patch.

That's true.

> In nodeHash.c, you missed an underscore:
> + * Target in-memory hashtable size is hashmem kilobytes.

Got it; thanks.

--
Peter Geoghegan