Horton

Re: Default setting for enable_hashagg_disk

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Jeff Davis <pgsql@j-davis.com>, David Rowley <dgrowleyml@gmail.com>, Stephen Frost <sfrost@snowman.net>, Tom Lane <tgl@sss.pgh.pa.us>, 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: 2020-07-15T04:12:35Z
Lists: pgsql-hackers, pgsql-docs

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On Mon, Jul 13, 2020 at 9:47 AM Alvaro Herrera <alvherre@2ndquadrant.com> wrote:
> I'm in favor of hash_mem_multiplier.  I think a >1 default is more
> sensible than =1 in the long run, but if strategic vote is what we're
> doing, then I support the =1 option.

Attached is a WIP patch implementing hash_mem_multiplier, with 1.0 as
the GUC's default value (i.e. the patch introduces no behavioral
changes by default). The first patch in the series renames some local
variables whose name is made ambiguous by the second, main patch.

Since the patch doesn't add a new work_mem-style GUC, but existing
consumers of work_mem expect something like that, the code is
structured in a way that allows the planner and executor to pretend
that there really is a work_mem-style GUC called hash_mem, which they
can determine the value of by calling the get_hash_mem() function.
This seemed like the simplest approach overall. I placed the
get_hash_mem() function in nodeHash.c, which is a pretty random place
for it. If anybody has any better ideas about where it should live,
please say so.

ISTM that the planner changes are where there's mostly likely to be
problems. Reviewers should examine consider_groupingsets_paths() in
detail.

--
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.