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: 2020-07-18T18:30:43Z
Lists: pgsql-hackers, pgsql-docs 
Jeff Davis <pgsql@j-davis.com> writes:
> On Fri, 2020-07-17 at 18:38 -0700, Peter Geoghegan wrote:
>> 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?

> 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.

I'm honestly a bit baffled about the level of fear being expressed
around this feature.  We have *frequently* made changes that would
change query plans, perhaps not 100.00% for the better, and never
before have we had this kind of bikeshedding about whether it was
necessary to be able to turn it off.  I think the entire discussion
is way out ahead of any field evidence that we need such a knob.
In the absence of evidence, our default position ought to be to
keep it simple, not to accumulate backwards-compatibility kluges.

(The only reason I'm in favor of heap_mem[_multiplier] is that it
seems like it might be possible to use it to get *better* plans
than before.  I do not see it as a backwards-compatibility knob.)

			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.