Horton

Re: Default setting for enable_hashagg_disk

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: David Rowley <dgrowleyml@gmail.com>, Stephen Frost <sfrost@snowman.net>, Tom Lane <tgl@sss.pgh.pa.us>, Jeff Davis <pgsql@j-davis.com>, Alvaro Herrera <alvherre@2ndquadrant.com>, 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: 2020-07-13T20:12:35Z
Lists: pgsql-hackers, pgsql-docs 
On Mon, Jul 13, 2020 at 12:57 PM David G. Johnston
<david.g.johnston@gmail.com> wrote:
> To be clear, by "escape hatch" you mean "add a GUC that instructs the PostgreSQL executor to ignore hash_mem when deciding whether to spill the contents of the hash table to disk - IOW to never spill the contents of a hash table to disk"?

Yes, that's what that means.

> If so that seems separate from whether to add a hash_mem GUC to provide finer grained control - people may well want both.

They might want the escape hatch too, as an additional measure, but my
assumption is that anybody in favor of the
hash_mem/hash_mem_multiplier proposal takes that position because they
think that it's the principled solution. That's the kind of subtlety
that is bound to get lost when summarizing general sentiment at a high
level. In any case no individual has seriously argued that there is a
simultaneous need for both -- at least not yet.

This thread is already enormous, and very hard to keep up with. I'm
trying to draw a line under the discussion. For my part, I have
compromised on the important question of the default value of
hash_mem_multiplier -- I am writing a new version of the patch that
makes the default 1.0 (i.e. no behavioral changes by default).

> I would prefer DavidJ as an abbreviation - my middle initial can be dropped when referring to me.

Sorry about that.

-- 
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.