Horton

Re: Default setting for enable_hashagg_disk

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Jeff Davis <pgsql@j-davis.com>
Cc: Bruce Momjian <bruce@momjian.us>, Andres Freund <andres@anarazel.de>, 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: 2020-06-25T17:47:51Z
Lists: pgsql-hackers, pgsql-docs 
On Thu, Jun 25, 2020 at 1:15 PM Jeff Davis <pgsql@j-davis.com> wrote:
> Unexpected things (meaning underestimates) are not independent. All the
> queries are based on the same stats, so if you have a lot of similar
> queries, they will all get the same underestimate at once, and all be
> surprised when they need to spill at once, and then all decide they are
> entitled to ignore work_mem at once.

Yeah, that's a risk. But what is proposed is a configuration setting,
so people can adjust it depending on what they think is likely to
happen in their environment.

> That sounds more useful and probably not too hard to implement in a
> crude form. Just have a shared counter in memory representing GB. If a
> node is about to spill, it could try to decrement the counter by N, and
> if it succeeds, it gets to exceed work_mem by N more GB.

That's a neat idea, although GB seems too coarse.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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.