Re: Default setting for enable_hashagg_disk

On Mon, 2020-06-22 at 10:52 -0400, Robert Haas wrote:
> So I feel like the really important thing here is to fix the cases
> that don't come out well with default settings.

...with the caveat that perfection is not something to expect from our
planner.

> If we can't do that,
> then the feature is half-baked and maybe should not have been
> committed in the first place.

HashAgg started out half-baked at the dawn of time, and stayed that way
through version 12. Disk-based HashAgg was designed to fix it.

Other major planner features generally offer a way to turn them off
(e.g. parallelism, JIT), and we don't call those half-baked.

I agree that the single GUC added in v13 (hashagg_avoid_disk_plan) is
weird because it's half of a disable switch. But it's not weird because
of my changes in v13; it's weird because the planner behavior in v12
was weird. I hope not many people need to set it, and I hope we can
remove it soon.

If you think we will never be able to remove the GUC, then we should
think a little harder about whether we really need it. I am open to
that discussion, but I don't think the presence of this GUC implies
that disk-based hashagg is half-baked.

Regards,
Jeff Davis