Re: Default setting for enable_hashagg_disk

On Fri, Jul 10, 2020 at 6:43 PM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Sat, 11 Jul 2020 at 13:36, David G. Johnston
> <david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
> > If we add a setting that defaults to work_mem then the benefit is
> severely reduced. You still have to modify individual queries, but the
> change can simply be more targeted than changing work_mem alone.
>
> I think the idea is that this is an escape hatch to allow users to get
> something closer to what PG12 did, but only if they really need it. I
> can't quite understand why we need to leave the escape hatch open and
> push them halfway through it. I find escape hatches are best left
> closed until you really have no choice but to use them.
>
>
The escape hatch dynamic is "the user senses a problem, goes into their
query, and modifies some GUCs to make the problem go away". As a user
I'd much rather have the odds of my needing to use that escape hatch
reduced - especially if that reduction can be done without risk and without
any action on my part.

It's like having someone in a box right now, and then turning up the heat.
We can give them an opening to get out of the box if they need it but we
can also give them A/C. For some the A/C may be unnecessary, but also not
harmful, while a smaller group will stay in the margin, while for the
others it's not enough and use the opening (which they would have done
anyway without the A/C).

David J.