Horton

Re: Default setting for enable_hashagg_disk

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jeff Davis <pgsql@j-davis.com>, 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: 2020-07-18T21:17:37Z
Lists: pgsql-hackers, pgsql-docs 
On Sat, Jul 18, 2020 at 11:30 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Jeff Davis <pgsql@j-davis.com> writes:
> > Yes, I think we should have that GUC (hashagg_avoid_disk_plan) for at
> > least one release.
>
> You'e being optimistic about it being possible to remove a GUC once
> we ship it.  That seems to be a hard sell most of the time.

You've said that you're +0.5 on removing this GUC, while Jeff seems to
be about -0.5 (at least that's my take). It's hard to see a way
towards closing out the hashagg_avoid_disk_plan open item if that's
our starting point.

The "do we need to keep hashagg_avoid_disk_plan?" question is
fundamentally a value judgement IMV. I believe that you both
understand each other's perspectives. I also suspect that no pragmatic
compromise will be possible -- we can either have the
hashagg_avoid_disk_plan GUC or not have it. ISTM that we're
deadlocked, at least in a technical or procedural sense.

Does that understanding seem accurate to you both?

-- 
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.