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: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>, Bruce Momjian <bruce@momjian.us>, David Rowley <dgrowleyml@gmail.com>, Jeff Davis <pgsql@j-davis.com>, Robert Haas <robertmhaas@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, pgsql-hackers@postgresql.org
Date: 2020-06-24T18:40:50Z
Lists: pgsql-hackers, pgsql-docs 
Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> On Wed, Jun 24, 2020 at 01:29:56PM -0400, Tom Lane wrote:
>> If we feel we need something to let people have the v12 behavior
>> back, let's have
>> (1) enable_hashagg on/off --- controls planner, same as it ever was
>> (2) enable_hashagg_spill on/off --- controls executor by disabling spill

> What if a user specifies
>    enable_hashagg = on
>    enable_hashagg_spill = off

It would probably be reasonable for the planner to behave as it did
pre-v13, that is not choose hashagg if it estimates that work_mem
would be exceeded.  (So, okay, that means enable_hashagg_spill
affects both planner and executor ... but ISTM it's just one
behavior not two.)

			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.