Horton

Re: [PoC] Reducing planning time when tables have many partitions

amit <amitlangote09@gmail.com>

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From: Amit Langote <amitlangote09@gmail.com>
To: David Rowley <dgrowleyml@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, Yuya Watari <watari.yuya@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Dmitry Dolgov <9erthalion6@gmail.com>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>, jian he <jian.universality@gmail.com>, Alena Rybakina <lena.ribackina@yandex.ru>, Andrei Lepikhov <a.lepikhov@postgrespro.ru>, Thom Brown <thom@linux.com>, Zhang Mingli <zmlpostgres@gmail.com>
Date: 2025-04-09T05:38:31Z
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. Update wording in optimizer/README for EquivalenceClasses

  2. Speedup child EquivalenceMember lookup in planner

  3. Allow planner to use Merge Append to efficiently implement UNION

  4. Remove trailing zero words from Bitmapsets

  5. Make Vars be outer-join-aware.

  6. Avoid making commutatively-duplicate clauses in EquivalenceClasses.

On Wed, Apr 9, 2025 at 2:21 PM David Rowley <dgrowleyml@gmail.com> wrote:
> On Wed, 9 Apr 2025 at 17:09, Amit Langote <amitlangote09@gmail.com> wrote:
> > Should the following paragraph in src/backend/optimizer/README be
> > updated to reflect the new reality after recent changes?
> >
> >     An EquivalenceClass can contain "em_is_child" members, which are copies
> >     of members that contain appendrel parent relation Vars, transposed to
> >     contain the equivalent child-relation variables or expressions. These
> >     members are not full-fledged members of the EquivalenceClass and do not
> >     affect the class's overall properties at all. They are kept only to
> >     simplify matching of child-relation expressions to EquivalenceClasses.
> >     Most operations on EquivalenceClasses should ignore child members.
> >
> > The part about these being in the EquivalenceClass might be worth
> > rewording now that we keep them in a separate array.
>
> I did read over that as part of the search I did for things that need
> to be updated, but I didn't see the need to adjust anything since the
> text doesn't talk about where the members are stored.  The only thing
> I see as a hint to that is the final sentence.
>
> If the README is light on documentation about where members are
> stored, do we really need to start detailing that because of this
> change?  I've tried to be fairly comprehensive about where members are
> stored in the header comment for struct EquivalenceClass. Wouldn't
> stating something similar in the README just be duplicating that?  I
> always think of the READMEs as more of an overview on how things fit
> together with some high-level theory. I think talking about data
> structures might be a bit too much detail.
>
> I'm happy to view wording suggestions if you think we need to detail
> this further. Maybe there's something that can be adjusted without
> going into too much depth.

Fair point that the current text isn't wrong (as Tom says) -- and we
do have the storage details in the struct comment already as you say.

Still, maybe a tiny tweak to the last line could help steer readers
right without diving into storage. How about:

Most operations on EquivalenceClasses should ignore child members,
which are stored separately from normal members.

No big deal either way -- just throwing it out there.

--
Thanks, Amit Langote