Horton

Re: Default setting for enable_hashagg_disk

Jeff Davis <pgsql@j-davis.com>

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From: Jeff Davis <pgsql@j-davis.com>
To: Andres Freund <andres@anarazel.de>, David Rowley <dgrowleyml@gmail.com>
Cc: Robert Haas <robertmhaas@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: 2020-06-25T16:24:52Z
Lists: pgsql-hackers, pgsql-docs 
On Wed, 2020-06-24 at 12:14 -0700, Andres Freund wrote:
> E.g. if the plan isn't expected to spill,
> only spill at 10 x work_mem or something like that.

Let's say you have work_mem=32MB and a query that's expected to use
16MB of memory. In reality, it uses 64MB of memory. So you are saying
this query would get to use all 64MB of memory, right?

But then you run ANALYZE. Now the query is (correctly) expected to use
64MB of memory. Are you saying this query, executed again with better
stats, would only get to use 32MB of memory, and therefore run slower?

Regards,
	Jeff Davis

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.