Horton

Re: POC: GROUP BY optimization

Alexander Korotkov <aekorotkov@gmail.com>

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From: Alexander Korotkov <aekorotkov@gmail.com>
To: Andrei Lepikhov <a.lepikhov@postgrespro.ru>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Richard Guo <guofenglinux@gmail.com>, Pavel Borisov <pashkin.elfe@gmail.com>, vignesh C <vignesh21@gmail.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>, Tomas Vondra <tomas.vondra@enterprisedb.com>, Teodor Sigaev <teodor@sigaev.ru>, David Rowley <dgrowleyml@gmail.com>, "a.rybakina" <a.rybakina@postgrespro.ru>
Date: 2024-05-31T00:12:28Z
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

Attachments

Hi!

On Thu, May 30, 2024 at 7:22 AM Andrei Lepikhov
<a.lepikhov@postgrespro.ru> wrote:
> On 5/29/24 19:53, Alexander Korotkov wrote:
> > Thank you for your feedback.
> >
> > On Wed, May 29, 2024 at 11:08 AM Andrei Lepikhov
> > <a.lepikhov@postgrespro.ru> wrote:
> >> On 5/27/24 19:41, Alexander Korotkov wrote:
> >>> Any thoughts?
> >> About 0001:
> >> Having overviewed it, I don't see any issues (but I'm the author),
> >> except grammatical ones - but I'm not a native to judge it.
> >> Also, the sentence 'turning GROUP BY clauses  into pathkeys' is unclear
> >> to me. It may be better to write something like:  'building pathkeys by
> >> the list of grouping clauses'.
> >
> > OK, thank you.  I'll run once again for the grammar issues.

I've revised some grammar including the sentence you've proposed.

> >> 0002:
> >> The part under USE_ASSERT_CHECKING looks good to me. But the code in
> >> group_keys_reorder_by_pathkeys looks suspicious: of course, we do some
> >> doubtful work without any possible way to reproduce, but if we envision
> >> some duplicated elements in the group_clauses, we should avoid usage of
> >> the list_concat_unique_ptr.
> >
> > As I understand Tom, there is a risk that clauses list may contain
> > multiple instances of equivalent SortGroupClause, not duplicate
> > pointers.
> Maybe. I just implemented the worst-case scenario with the intention of
> maximum safety. So, it's up to you.
> >
> >> What's more, why do you not exit from
> >> foreach_ptr immediately after SortGroupClause has been found? I think
> >> the new_group_clauses should be consistent with the new_group_pathkeys.
> >
> > I wanted this to be consistent with preprocess_groupclause(), where
> > duplicate SortGroupClause'es are grouped together.  Otherwise, we
> > could just delete redundant SortGroupClause'es.
> Hm, preprocess_groupclause is called before the standard_qp_callback
> forms the 'canonical form' of group_pathkeys and such behaviour needed.
> But the code that chooses the reordering strategy uses already processed
> grouping clauses, where we don't have duplicates in the first
> num_groupby_pathkeys elements, do we?

Yep, it seems that we work with group clauses which were processed by
standard_qp_callback().  In turn standard_qp_callback() called
make_pathkeys_for_sortclauses_extended() with remove_redundant ==
true.  So, there shouldn't be duplicates.  And it's unclear what
should we be protected from.

I leaved 0002 with just asserts.  And extracted questionable changes into 0005.

------
Regards,
Alexander Korotkov
Supabase