Horton

Re: Default setting for enable_hashagg_disk

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Jeff Davis <pgsql@j-davis.com>
Cc: David Rowley <dgrowleyml@gmail.com>, 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: 2020-06-25T16:37:46Z
Lists: pgsql-hackers, pgsql-docs 
On 2020-06-25 09:24:52 -0700, Jeff Davis wrote:
> 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?

Yes. I think that's ok, because it was taken into account from a costing
perspective int he second case.

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.