On Thu, Jun 25, 2020 at 12:24:29AM +1200, David Rowley wrote:
> On Wed, 24 Jun 2020 at 21:06, Bruce Momjian <bruce@momjian.us> wrote:
> > I
> > don't remember anyone complaining about spills to disk during merge
> > join, so I am unclear why we would need a such control for hash join.
>
> Hash aggregate, you mean? The reason is that upgrading to PG13 can
Yes, sorry.
> cause a performance regression for an underestimated ndistinct on the
> GROUP BY clause and cause hash aggregate to spill to disk where it
> previously did everything in RAM. Sure, that behaviour was never
> what we wanted to happen, Jeff has fixed that now, but the fact
> remains that this does happen in the real world quite often and people
> often get away with it, likey because work_mem is generally set to
> some very conservative value. Of course, there's also a bunch of
> people that have been bitten by OOM due to this too. The "neverspill"
> wouldn't be for those people. Certainly, it's possible that we just
> tell these people to increase work_mem for this query, that way they
> can set it to something reasonable and still get spilling if it's
> really needed to save them from OOM, but the problem there is that
> it's not very easy to go and set work_mem for a certain query.
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.
--
Bruce Momjian <bruce@momjian.us> https://momjian.us
EnterpriseDB https://enterprisedb.com
The usefulness of a cup is in its emptiness, Bruce Lee