Horton

Re: Default setting for enable_hashagg_disk

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Jeff Davis <pgsql@j-davis.com>, David Rowley <dgrowleyml@gmail.com>, Stephen Frost <sfrost@snowman.net>, Peter Geoghegan <pg@bowt.ie>, Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Bruce Momjian <bruce@momjian.us>, Robert Haas <robertmhaas@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, pgsql-hackers@postgresql.org
Date: 2020-07-13T18:43:16Z
Lists: pgsql-hackers, pgsql-docs 
On Mon, Jul 13, 2020 at 12:47:36PM -0400, Alvaro Herrera wrote:
> On 2020-Jul-13, Jeff Davis wrote:
> 
> > On Tue, 2020-07-14 at 02:25 +1200, David Rowley wrote:
> > > Updated summary:
> > > * For hash_mem = Tomas [7], Justin [16]
> > > * For hash_mem_multiplier with a default > 1.0 = DavidG [21]
> > > * For hash_mem_multiplier with default = 1.0 =  PeterG [15][0], Tom
> > > [20][24]
> > 
> > I am OK with these options, but I still prefer a simple escape hatch.
> 
> I'm in favor of hash_mem_multiplier.  I think a >1 default is more
> sensible than =1 in the long run, but if strategic vote is what we're
> doing, then I support the =1 option.

I recanted and support hash_mem_multiplier (or something supporting that
behavior, even if it also supports an absolute/scalar value).
https://www.postgresql.org/message-id/20200703145620.GK4107@telsasoft.com

1.0 (or -1) is fine, possibly to be >= 1.0 in master at a later date.

-- 
Justin

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.