Horton

Re: Default setting for enable_hashagg_disk

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.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: 2020-07-18T01:38:02Z
Lists: pgsql-hackers, pgsql-docs 
On Fri, Jul 17, 2020 at 5:13 PM Jeff Davis <pgsql@j-davis.com> wrote:
> The idea is growing on me a bit. It doesn't give exactly v12 behavior,
> but it does offer another lever that might tackle a lot of the
> practical cases.

Cool.

> If I were making the decision alone, I'd still choose
> the escape hatch based on simplicity, but I'm fine with this approach
> as well.

There is also the separate question of what to do about the
hashagg_avoid_disk_plan GUC (this is a separate open item that
requires a separate resolution). Tom leans slightly towards removing
it now. Is your position about the same as before?

> The patch itself looks reasonable to me. I don't see a lot of obvious
> dangers, but perhaps someone would like to take a closer look at the
> planner changes as you suggest.

It would be good to get further input on the patch from somebody else,
particularly the planner aspects.

My intention is to commit the patch myself. I was the primary advocate
for hash_mem_multiplier, so it seems as if I should own it. (You may
have noticed that I just pushed the preparatory
local-variable-renaming patch, to get that piece out of the way.)

-- 
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.