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: 2020-07-07T09:50:26Z
Lists: pgsql-hackers, pgsql-docs 
On Tue, Jul 7, 2020 at 9:18 AM Jeff Davis <pgsql@j-davis.com> wrote:
>
> On Mon, 2020-07-06 at 15:59 +0530, Amit Kapila wrote:
> > 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?
>
> Perhaps "can't" was too strong of a word, but I think it would be
> unprecedented to introduce a GUC in a minor version.
>

I don't think this is true.  We seem to have introduced three new guc
variables in a 9.3.3 minor release.  See the following entry in 9.3.3
release notes [1]: "Create separate GUC parameters to control
multixact freezing....  Introduce new settings
vacuum_multixact_freeze_min_age, vacuum_multixact_freeze_table_age,
and autovacuum_multixact_freeze_max_age to control when to freeze
multixacts."

Apart from this, we have asked users to not only edit postgresql.conf
file but also update system catalogs.  See the fix for "Cope with the
Windows locale named "Norwegian (Bokmål)" [2] in 9.4.1 release.

There are other instances where we also suggest users to set gucs,
create new system objects (like views), perform DDL, DMLs, run REINDEX
on various indexes, etc. in the minor release.

[1] - https://www.postgresql.org/docs/release/9.3.3/
[2] - https://wiki.postgresql.org/wiki/Changes_To_Norwegian_Locale

-- 
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.