Re: Default setting for enable_hashagg_disk

On Saturday, July 11, 2020, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> "David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> > On Sat, Jul 11, 2020 at 5:47 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> It seems like a lot of the disagreement here is focused on Peter's
> >> proposal to make hash_mem_multiplier default to 2.0. But it doesn't
> >> seem to me that that's a critical element of the proposal. Why not just
> >> make it default to 1.0, thus keeping the default behavior identical
> >> to what it is now?
>
> > If we don't default it to something other than 1.0 we might as well just
> > make it memory units and let people decide precisely what they want to
> use
> > instead of adding the complexity of a multiplier.
>
> Not sure how that follows? The advantage of a multiplier is that it
> tracks whatever people might do to work_mem automatically.

>
I was thinking that setting -1 would basically do that.

> In general
> I'd view work_mem as the base value that people twiddle to control
> executor memory consumption. Having to also twiddle this other value
> doesn't seem especially user-friendly.

I’ll admit I don’t have a feel for what is or is not user-friendly when
setting these GUCs in a session to override the global defaults. But as
far as the global defaults I say it’s a wash between (32mb, -1) -> (32mb,
48mb) and (32mb, 1.0) -> (32mb, 1.5)

If you want 96mb for the session/query hash setting it to 96mb is
invariant, whilesetting it to 3.0 means it can change in the future if the
system work_mem changes. Knowing the multiplier is 1.5 and choosing 64mb
for work_mem in the session is possible but also mutable and has
side-effects. If the user is going to set both values to make it invariant
we are back to it being a wash.

I don’t believe using a multiplier will promote better comprehension for
why this setting exists compared to “-1 means use work_mem but you can
override a subset if you want.”

Is having a session level memory setting be mutable something we want to
introduce?

Is it more user-friendly?

>> If we find that's a poor default, we can always change it later;
> >> but it seems to me that the evidence for a higher default is
> >> a bit thin at this point.
>
> > So "your default is 1.0 unless you installed the new database on or after
> > 13.4 in which case it's 2.0"?
>
> What else would be new? See e.g. 848ae330a. (Note I'm not suggesting
> that we'd change it in a minor release.)
>

Minor release update is what I had thought, and to an extent was making
possible by not using the multiplier upfront.

I agree options are wide open come v14 and beyond.

David J.