Horton

Re: Default setting for enable_hashagg_disk

Bruce Momjian <bruce@momjian.us>

View thread View on postgr.es
From: Bruce Momjian <bruce@momjian.us>
To: Justin Pryzby <pryzby@telsasoft.com>
Cc: David Rowley <dgrowleyml@gmail.com>, Jeff Davis <pgsql@j-davis.com>, Robert Haas <robertmhaas@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, pgsql-hackers@postgresql.org
Date: 2020-06-24T17:08:34Z
Lists: pgsql-hackers, pgsql-docs 
On Wed, Jun 24, 2020 at 07:38:43AM -0500, Justin Pryzby wrote:
> On Wed, Jun 24, 2020 at 05:06:28AM -0400, Bruce Momjian wrote:
> > It would seem merge join has almost the same complexities as the new
> > hash join code, since it can spill to disk doing sorts for merge joins,
> > and adjusting work_mem is the only way to control that spill to disk.  I
> > don't remember anyone complaining about spills to disk during merge
> > join, so I am unclear why we would need a such control for hash join.
> 
> It loooks like merge join was new in 8.3.  I don't think that's a good analogy,
> since the old behavior was still available with enable_mergejoin=off.

Uh, we don't gurantee backward compatibility in the optimizer.  You can
turn off hashagg if you want.  That doesn't get you to PG 13 behavior,
but we don't gurantee that.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee

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.