Re: Default setting for enable_hashagg_disk

Jeff Davis <pgsql(at)j-davis(dot)com> writes:
> On Fri, 2020-07-17 at 18:38 -0700, Peter Geoghegan wrote:
>> There is also the separate question of what to do about the
>> hashagg_avoid_disk_plan GUC (this is a separate open item that
>> requires a separate resolution). Tom leans slightly towards removing
>> it now. Is your position about the same as before?

> Yes, I think we should have that GUC (hashagg_avoid_disk_plan) for at
> least one release.

You'e being optimistic about it being possible to remove a GUC once
we ship it. That seems to be a hard sell most of the time.

I'm honestly a bit baffled about the level of fear being expressed
around this feature. We have *frequently* made changes that would
change query plans, perhaps not 100.00% for the better, and never
before have we had this kind of bikeshedding about whether it was
necessary to be able to turn it off. I think the entire discussion
is way out ahead of any field evidence that we need such a knob.
In the absence of evidence, our default position ought to be to
keep it simple, not to accumulate backwards-compatibility kluges.

(The only reason I'm in favor of heap_mem[_multiplier] is that it
seems like it might be possible to use it to get *better* plans
than before. I do not see it as a backwards-compatibility knob.)

regards, tom lane