Re: Default setting for enable_hashagg_disk

On Mon, 2020-06-22 at 15:28 -0400, Robert Haas wrote:
> The weirdness is the problem here, at least for me. Generally, I
> don't
> like GUCs of the form give_me_the_old_strange_behavior=true

I agree with all of that in general.

> I don't think it necessarily implies that either. I do however have
> some concerns about people using the GUC as a crutch.

Another way of looking at it is that the weird behavior is already
there in v12, so there are already users relying on this weird behavior
as a crutch for some other planner mistake. The question is whether we
want to:

(a) take the weird behavior away now as a consequence of implementing
disk-based HashAgg; or
(b) support the weird behavior forever; or
(c) introduce a GUC now to help transition away from the weird behavior

The danger with (c) is that it gives users more time to become more
reliant on the weird behavior; and worse, a GUC could be seen as an
endorsement of the weird behavior rather than a path to eliminating it.
So we could intend to do (c) and end up with (b). We can mitigate this
with documentation warnings, perhaps.

> I am slightly
> worried that this is going to have hard-to-fix problems and that
> we'll
> be stuck with the GUC for that reason.

Without the GUC, it's basically a normal cost-based decision, with all
of the good and bad that comes with that.

> Now if that is the case, is
> removing the GUC any better? Maybe not. These decisions are hard, and
> I am not trying to pretend like I have all the answers.

I agree that there is no easy answer.

My philosophy here is: if a user does experience a plan regression due
to my change, would it be reasonable to tell them that we don't have
any escape hatch or transition period at all? That would be a tough
sell for such a common plan type.

Regards,
Jeff Davis