Re: Default setting for enable_hashagg_disk

On Wed, Jun 24, 2020 at 07:18:10PM -0400, Bruce Momjian wrote:
> On Wed, Jun 24, 2020 at 12:19:00PM -0700, Andres Freund wrote:
> > Hi,
> >
> > On 2020-06-24 13:12:03 -0400, Bruce Momjian wrote:
> > > Well, my point is that merge join works that way, and no one has needed
> > > a knob to avoid mergejoin if it is going to spill to disk. If they are
> > > adjusting work_mem to prevent spill of merge join, they can do the same
> > > for hash agg. We just need to document this in the release notes.
> >
> > I don't think this is comparable. For starters, the IO indirectly
> > triggered by mergejoin actually leads to plenty people just straight out
> > disabling it. For lots of workloads there's never a valid reason to use
> > a mergejoin (and often the planner will never choose one). Secondly, the
> > planner has better information about estimating the memory usage for the
> > to-be-sorted data than it has about the size of the transition
> > values. And lastly, there's a difference between a long existing cause
> > for bad IO behaviour and one that's suddenly kicks in after a major
> > version upgrade, to which there's no escape hatch (it's rarely realistic
> > to disable hash aggs, in contrast to merge joins).
>
> Well, this sounds like an issue of degree, rather than kind. It sure
> sounds like "ignore work_mem for this join type, but not the other".

I think my main point is that work_mem was not being honored for
hash-agg before, but now that PG 13 can do it, we are again allowing
work_mem not to apply in certain cases. I am wondering if our hard
limit for work_mem is the issue, and we should make that more flexible
for all uses.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EnterpriseDB https://enterprisedb.com

The usefulness of a cup is in its emptiness, Bruce Lee