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: 2022-07-15T05:18:09Z
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 still working in this area and I noticed that db0d67db2 updated
some regression tests in partition_aggregate.out without any care as
to what the test was testing.

The comment above the test reads:

-- Without ORDER BY clause, to test Gather at top-most path

and you've changed the expected plan from being a parallel plan with a
Gather to being a serial plan. So it looks like the test might have
become useless.

I see that the original plan appears to come back with some
adjustments to parallel_setup_cost and parallel_tuple_cost. It seems a
bit strange to me that the changes with this patch would cause a
change of plan for this. There is only 1 GROUP BY column in the query
in question. There's no rearrangement to do with a single column GROUP
BY.

David