Re: Removing unneeded self joins

Bruce Momjian <bruce@momjian.us>

From: Bruce Momjian <bruce@momjian.us>

To: Robert Haas <robertmhaas@gmail.com>

Cc: Andrei Lepikhov <a.lepikhov@postgrespro.ru>, Alexander Korotkov <aekorotkov@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, Richard Guo <guofenglinux@gmail.com>, Alexander Lakhin <exclusion@gmail.com>, "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Michał Kłeczek <michal@kleczek.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2024-05-06T17:08:35Z

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 →

Remove GUC_NOT_IN_SAMPLE from enable_self_join_elimination
- 717d0e8dd945 18.0 landed
Put enable_self_join_elimination into postgresql.conf.sample
- c2d329260cd8 18.0 landed
Get rid of ojrelid local variable in remove_rel_from_query()
- e167191dc146 18.0 landed
Implement Self-Join Elimination
- fc069a3a6319 18.0 cited
Revert: Remove useless self-joins
- d1d286d83c0e 17.0 landed
Replace lateral references to removed rels in subqueries
- 466979ef031a 17.0 landed
Replace relids in lateral subquery parse tree during SJE
- 489072ab7a9e 17.0 landed
Forbid SJE with result relation
- 8c441c082797 17.0 landed
Fix misuse of RelOptInfo.unique_for_rels cache by SJE
- 30b4955a4668 17.0 landed
Replace the relid in some missing fields during SJE
- a7928a57b9f0 17.0 landed
Revert 56-bit relfilenode change and follow-up commits.
- a448e49bcbe4 16.0 cited
Stabilize timetz test across DST transitions.
- 4a071afbd056 14.0 cited
Speed up finding EquivalenceClasses for a given set of rels
- 3373c7155350 13.0 cited
Fix mark-and-restore-skipping test case to not be a self-join.
- 24d08f3c0a1f 12.0 landed

On Mon, May  6, 2024 at 12:24:41PM -0400, Robert Haas wrote:
> Now that being said, I do also agree that the planner code is quite
> hard to understand, for various reasons. I don't think the structure
> of that code and the assumptions underlying it are as well-documented
> as they could be, and neither do I think that all of them are optimal.
> It has taken me a long time to learn as much as I know, and there is
> still quite a lot that I don't know. And I also agree that the planner
> does an unfortunate amount of in-place modification of existing
> structures without a lot of clarity about how it all works, and an
> unfortunate amount of data copying in some places, and even that the
> partition-wise join code isn't all that it could be. But I do not
> think that adds up to a conclusion that we should just be less
> ambitious with planner changes. Indeed, I would like to see us do
> more. There is certainly a lot of useful work that could be done. The
> trick is figuring out how to do it without breaking too many things,
> and that is not easy.

I agree with Robert.  While writting the Postgres 17 release notes, I am
excited to see the many optimizer improvements, and removing self-joins
from that list will be unfortunate.

I did write a blog entry in 2021 that suggested we could have
optimizer aggressiveness control to allow for more expensive
optimizations:

	https://momjian.us/main/blogs/pgblog/2021.html#May_14_2021

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.