Re: Default setting for enable_hashagg_disk

On Thu, Jun 25, 2020 at 1:15 PM Jeff Davis <pgsql(at)j-davis(dot)com> wrote:
> Unexpected things (meaning underestimates) are not independent. All the
> queries are based on the same stats, so if you have a lot of similar
> queries, they will all get the same underestimate at once, and all be
> surprised when they need to spill at once, and then all decide they are
> entitled to ignore work_mem at once.

Yeah, that's a risk. But what is proposed is a configuration setting,
so people can adjust it depending on what they think is likely to
happen in their environment.

> That sounds more useful and probably not too hard to implement in a
> crude form. Just have a shared counter in memory representing GB. If a
> node is about to spill, it could try to decrement the counter by N, and
> if it succeeds, it gets to exceed work_mem by N more GB.

That's a neat idea, although GB seems too coarse.

--
Robert Haas
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