Horton

Re: Default setting for enable_hashagg_disk

Jeff Davis <pgsql@j-davis.com>

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From: Jeff Davis <pgsql@j-davis.com>
To: Justin Pryzby <pryzby@telsasoft.com>, Stephen Frost <sfrost@snowman.net>
Cc: Peter Geoghegan <pg@bowt.ie>, Alvaro Herrera <alvherre@2ndquadrant.com>, Andres Freund <andres@anarazel.de>, Bruce Momjian <bruce@momjian.us>, Robert Haas <robertmhaas@gmail.com>, David Rowley <dgrowleyml@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, pgsql-hackers@postgresql.org
Date: 2020-07-10T08:29:18Z
Lists: pgsql-hackers, pgsql-docs 
On Thu, 2020-07-09 at 19:18 -0500, Justin Pryzby wrote:
> Maybe pretend that Jeff implemented something called CashAgg, which
> does
> everything HashAgg does but implemented from scratch.  Users would be
> able to
> tune it or disable it, and we could talk about removing HashAgg for
> the next 3
> years.

That's kind of what we'd have if we had the two escape-hatch GUCs.
Default gives new behavior, changing the GUCs would give the v12
behavior.

In principle, Stephen is right: the v12 behavior is a bug, lots of
people are unhappy about it, it causes real problems, and it would not
be acceptable if proposed today. Otherwise I wouldn't have spent the
time to fix it. 

Similarly, potential regressions are not the "fault" of my feature --
they are the fault of the limitations of work_mem, the limitations of
the planner, the wrong expectations from customers, or just
happenstance. 

But at a certain point, I have to weigh the potential anger of
customers hitting regressions versus the potential anger of hackers
seeing a couple extra GUCs. I have to say that I am more worried about
the former.

If there is some more serious consequence of adding a GUC that I missed
in this thread, please let me know. Otherwise, I intend to commit a new
GUC shortly that will enable users to bypass work_mem for HashAgg, just
as in v12.

Regards,
	Jeff Davis

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.