Re: Default setting for enable_hashagg_disk

On Fri, Jul 10, 2020 at 10:47 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I looked over Peter's patch in [1], and it seems generally pretty
> sane to me, though I concur with the idea that it'd be better to
> define the GUC as a multiplier for work_mem. (For one thing, we could
> then easily limit it to be at least 1.0, ensuring sanity; also, if
> work_mem does eventually become more dynamic than it is now, we might
> still be able to salvage this knob as something useful. Or if not,
> we just rip it out.) So my vote is for moving in that direction.

Cool. I agree that it makes sense to constrain the effective value to
be at least work_mem in all cases.

With that in mind, I propose that this new GUC have the following
characteristics:

* It should be named "hash_mem_multiplier", a floating point GUC
(somewhat like bgwriter_lru_multiplier).

* The default value is 2.0.

* The minimum allowable value is 1.0, to protect users from
accidentally giving less memory to hash-based nodes.

* The maximum allowable value is 100.0, to protect users from
accidentally setting hash_mem_multiplier to a value intended to work
like a work_mem-style KB value (you can't provide an absolute value
like that directly). This maximum is absurdly high.

I think that it's possible that a small number of users will find it
useful to set the value of hash_mem_multiplier as high as 5.0. That is
a very aggressive value, but one that could still make sense with
certain workloads.

Thoughts?
--
Peter Geoghegan