Horton

Re: POC: GROUP BY optimization

David Rowley <dgrowleyml@gmail.com>

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From: David Rowley <dgrowleyml@gmail.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Zhihong Yu <zyu@yugabyte.com>, "Andrey V. Lepikhov" <a.lepikhov@postgrespro.ru>, Teodor Sigaev <teodor@sigaev.ru>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2022-07-13T03:37:19Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Restore preprocess_groupclause()

  2. Rename PathKeyInfo to GroupByOrdering

  3. Add invariants check to get_useful_group_keys_orderings()

  4. Fix asymmetry in setting EquivalenceClass.ec_sortref

  5. Multiple revisions to the GROUP BY reordering tests

  6. Get rid of pg_class usage in SJE regression tests

  7. Rename index "abc" in aggregates.sql

  8. Explore alternative orderings of group-by pathkeys during optimization.

  9. Generalize the common code of adding sort before processing of grouping

  10. Fix out-dated comment in preprocess_groupclause()

  11. Force parallelism in partition_aggregate

  12. Optimize order of GROUP BY keys

On Thu, 31 Mar 2022 at 12:19, Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
> Pushed, after going through the patch once more, running check-world
> under valgrind, and updating the commit message.

I'm just in this general area of the code again today and wondered
about the header comment for the preprocess_groupclause() function.

It says:

 * In principle it might be interesting to consider other orderings of the
 * GROUP BY elements, which could match the sort ordering of other
 * possible plans (eg an indexscan) and thereby reduce cost.  We don't
 * bother with that, though.  Hashed grouping will frequently win anyway.

I'd say this commit makes that paragraph mostly obsolete.  It's only
true now in the sense that we don't try orders that suit some index
that would provide pre-sorted results for a GroupAggregate path.  The
comment leads me to believe that we don't do anything at all to find a
better order, and that's not true now.

David