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: Peter Geoghegan <pg@bowt.ie>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>, David Rowley <dgrowleyml@gmail.com>, Stephen Frost <sfrost@snowman.net>, Tom Lane <tgl@sss.pgh.pa.us>, Andres Freund <andres@anarazel.de>, Bruce Momjian <bruce@momjian.us>, 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: 2020-07-18T18:16:26Z
Lists: pgsql-hackers, pgsql-docs 
On Fri, 2020-07-17 at 18:38 -0700, Peter Geoghegan wrote:
> There is also the separate question of what to do about the
> hashagg_avoid_disk_plan GUC (this is a separate open item that
> requires a separate resolution). Tom leans slightly towards removing
> it now. Is your position about the same as before?

Yes, I think we should have that GUC (hashagg_avoid_disk_plan) for at
least one release.

Clealy, a lot of plans will change. For any GROUP BY where there are a
lot of groups, there was only one choice in v12 and now there are two
choices in v13. Obviously I think most of those changes will be for the
better, but some regressions are bound to happen. Giving users some
time to adjust, and for us to tune the cost model based on user
feedback, seems prudent.

Are there other examples of widespread changes in plans where we
*didn't* have a GUC? There are many GUCs for controlling parallism,
JIT, etc.

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.