Re: Default setting for enable_hashagg_disk

On Wed, Jun 24, 2020 at 3:14 PM Andres Freund <andres@anarazel.de> wrote:
> FWIW, my gut feeling is that we'll end up have to separate the
> "execution time" spilling from using plain work mem, because it'll
> trigger spilling too often. E.g. if the plan isn't expected to spill,
> only spill at 10 x work_mem or something like that.  Or we'll need
> better management of temp file data when there's plenty memory
> available.

So, I don't think we can wire in a constant like 10x. That's really
unprincipled and I think it's a bad idea. What we could do, though, is
replace the existing Boolean-valued GUC with a new GUC that controls
the size at which the aggregate spills. The default could be -1,
meaning work_mem, but a user could configure a larger value if desired
(presumably, we would just treat a value smaller than work_mem as
work_mem, and document the same).

I think that's actually pretty appealing. Separating the memory we
plan to use from the memory we're willing to use before spilling seems
like a good idea in general, and I think we should probably also do it
in other places - like sorts.

-- 
Robert Haas
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