Horton

Re: Default setting for enable_hashagg_disk (hash_mem)

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Bruce Momjian <bruce@momjian.us>, Peter Geoghegan <pg@bowt.ie>, Justin Pryzby <pryzby@telsasoft.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Andres Freund <andres@anarazel.de>, David Rowley <dgrowleyml@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2020-07-06T10:29:29Z
Lists: pgsql-hackers, pgsql-docs 
On Sun, Jul 5, 2020 at 2:24 AM Jeff Davis <pgsql@j-davis.com> wrote:
>
> On Sat, 2020-07-04 at 14:49 +0530, Amit Kapila wrote:
> > > We don't even have a user report yet of a
> > > regression compared to PG 12, or one that can't be fixed by
> > > increasing
> > > work_mem.
> > >
> >
> > Yeah, this is exactly the same point I have raised above.  I feel we
> > should wait before designing any solution to match pre-13 behavior
> > for
> > hashaggs to see what percentage of users face problems related to
> > this
> > and how much is a problem for them to increase work_mem to avoid
> > regression.
>
> I agree that it's good to wait for actual problems. But the challenge
> is that we can't backport an added GUC.
>

Is it because we won't be able to edit existing postgresql.conf file
or for some other reasons?

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com

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.