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: "David G. Johnston" <david.g.johnston@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Stephen Frost <sfrost@snowman.net>, Peter Geoghegan <pg@bowt.ie>, 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>, David Rowley <dgrowleyml@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, Melanie Plageman <melanieplageman@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-07-12T04:37:08Z
Lists: pgsql-hackers, pgsql-docs 
"David G. Johnston" <david.g.johnston@gmail.com> writes:
> On Sat, Jul 11, 2020 at 5:47 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> It seems like a lot of the disagreement here is focused on Peter's
>> proposal to make hash_mem_multiplier default to 2.0.  But it doesn't
>> seem to me that that's a critical element of the proposal.  Why not just
>> make it default to 1.0, thus keeping the default behavior identical
>> to what it is now?

> If we don't default it to something other than 1.0 we might as well just
> make it memory units and let people decide precisely what they want to use
> instead of adding the complexity of a multiplier.

Not sure how that follows?  The advantage of a multiplier is that it
tracks whatever people might do to work_mem automatically.  In general
I'd view work_mem as the base value that people twiddle to control
executor memory consumption.  Having to also twiddle this other value
doesn't seem especially user-friendly.

>> If we find that's a poor default, we can always change it later;
>> but it seems to me that the evidence for a higher default is
>> a bit thin at this point.

> So "your default is 1.0 unless you installed the new database on or after
> 13.4 in which case it's 2.0"?

What else would be new?  See e.g. 848ae330a.  (Note I'm not suggesting
that we'd change it in a minor release.)

			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.