Horton

Re: Default setting for enable_hashagg_disk

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: "David G. Johnston" <david.g.johnston@gmail.com>, Stephen Frost <sfrost@snowman.net>, Peter Geoghegan <pg@bowt.ie>, Jeff Davis <pgsql@j-davis.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, Andres Freund <andres@anarazel.de>, Bruce Momjian <bruce@momjian.us>, Robert Haas <robertmhaas@gmail.com>, David Rowley <dgrowleyml@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Melanie Plageman <melanieplageman@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-07-12T00:47:54Z
Lists: pgsql-hackers, pgsql-docs 
Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> I don't know, but one of the main arguments against simply suggesting
> people to bump up work_mem (if they're hit by the hashagg spill in v13)
> was that it'd increase overall memory usage for them. It seems strange
> to then propose a new GUC set to a default that would result in higher
> memory usage *for everyone*.

It seems like a lot of the disagreement here is focused on Peter's
proposal to make hash_mem_multiplier default to 2.0.  But it doesn't
seem to me that that's a critical element of the proposal.  Why not just
make it default to 1.0, thus keeping the default behavior identical
to what it is now?

If we find that's a poor default, we can always change it later;
but it seems to me that the evidence for a higher default is
a bit thin at this point.

			regards, tom lane

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.