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: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Stephen Frost <sfrost@snowman.net>, Jeff Davis <pgsql@j-davis.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>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2020-07-10T21:28:15Z
Lists: pgsql-hackers, pgsql-docs 
Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> On 2020-Jul-10, Peter Geoghegan wrote:
>> * The maximum allowable value is 100.0, to protect users from
>> accidentally setting hash_mem_multiplier to a value intended to work
>> like a work_mem-style KB value (you can't provide an absolute value
>> like that directly). This maximum is absurdly high.

> I'm not sure about this bit; sounds a bit like what has been qualified
> as "nannyism" elsewhere.  Suppose I want to give a hash table 2GB of
> memory for whatever reason.  If my work_mem is default (4MB) then I
> cannot possibly achieve that without altering both settings.
> So I propose that maybe we do want a maximum value, but if so it should
> be higher than what you propose.  I think 10000 is acceptable in that it
> doesn't get in the way.

I was kind of thinking 1000 as the limit ;-).  In any case, the code
will need to internally clamp the product to not exceed whatever the
work_mem physical limit is these days.

			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.