Horton

Re: Default setting for enable_hashagg_disk

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: David Rowley <dgrowleyml@gmail.com>, Peter Geoghegan <pg@bowt.ie>, "David G. Johnston" <david.g.johnston@gmail.com>, Stephen Frost <sfrost@snowman.net>, 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>, 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: 2020-07-12T02:46:10Z
Lists: pgsql-hackers, pgsql-docs 
On Sun, Jul 12, 2020 at 2:27 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> David Rowley <dgrowleyml@gmail.com> writes:
> > hmm yeah. It's unfortunate, but I'm not sure how I'd have implemented
> > it differently.  The problem is made worse by the fact that we'll only
> > release the memory for the hash table during ExecEndHashJoin(). If the
> > planner had some ability to provide the executor with knowledge that
> > the node would never be rescanned, then the executor could release the
> > memory for the hash table after the join is complete.
>
> EXEC_FLAG_REWIND seems to fit the bill already?

FWIW I have a patch that does exactly that, which I was planning to
submit for CF2 along with some other patches that estimate and measure
peak executor memory usage.

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.