Horton

Re: Default setting for enable_hashagg_disk

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Alvaro Herrera <alvherre@2ndquadrant.com>, David Rowley <dgrowleyml@gmail.com>, Stephen Frost <sfrost@snowman.net>, Andres Freund <andres@anarazel.de>, Bruce Momjian <bruce@momjian.us>, Robert Haas <robertmhaas@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Melanie Plageman <melanieplageman@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-07-19T21:23:11Z
Lists: pgsql-hackers, pgsql-docs 
On Sat, Jul 18, 2020 at 3:04 PM Jeff Davis <pgsql@j-davis.com> wrote:
> > 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.
>
> Fair enough. I think that was where Stephen and Amit were coming from,
> as well.

> That would lessen the number of changed plans, but we could easily
> remove the pessimization without controversy later if it turned out to
> be unnecessary, or if we further optimize HashAgg IO.

Does this mean that we've reached a final conclusion on
hashagg_avoid_disk_plan for Postgres 13, which is that it should be
removed? If so, I'd appreciate it if you took care of it. I don't
think that we need to delay its removal until the details of the
HashAgg cost pessimization are finalized. (I expect that that will be
totally uncontroversial.)

Thanks
-- 
Peter Geoghegan

Commits

  1. Add hash_mem_multiplier GUC.

  2. HashAgg: use better cardinality estimate for recursive spilling.

  3. Remove hashagg_avoid_disk_plan GUC.

  4. Doc fixup for hashagg_avoid_disk_plan GUC.

  5. Rework HashAgg GUCs.

  6. Disk-based Hash Aggregation.

  7. Implement partition-wise grouping/aggregation.

  8. Defer creation of partially-grouped relation until it's needed.