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: Jeff Davis <pgsql@j-davis.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Alvaro Herrera <alvherre@2ndquadrant.com>, David Rowley <dgrowleyml@gmail.com>, Stephen Frost <sfrost@snowman.net>, 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: 2020-07-19T01:15:33Z
Lists: pgsql-hackers, pgsql-docs 
Jeff Davis <pgsql@j-davis.com> writes:
> What is your opinion about pessimizing the HashAgg disk costs (not
> affecting HashAgg plans expected to stay in memory)? Tomas Vondra
> presented some evidence that Sort had some better IO patterns in some
> cases that weren't easily reflected in a principled way in the cost
> model.

Hm, was that in some other thread?  I didn't find any such info
in a quick look through this one.

> That would lessen the number of changed plans, but we could easily
> remove the pessimization without controversy later if it turned out to
> be unnecessary, or if we further optimize HashAgg IO.

Trying to improve our cost models under-the-hood seems like a
perfectly reasonable activity to me.

			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.