Re: Removing unneeded self joins

Andrei Lepikhov <lepihov@gmail.com>

From: Andrei Lepikhov <lepihov@gmail.com>

To: Michael Paquier <michael@paquier.xyz>, Alexander Korotkov <aekorotkov@gmail.com>

Cc: Alena Rybakina <a.rybakina@postgrespro.ru>, Richard Guo <guofenglinux@gmail.com>, Ranier Vilela <ranier.vf@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, Alexander Lakhin <exclusion@gmail.com>, "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Michał Kłeczek <michal@kleczek.org>, Dean Rasheed <dean.a.rasheed@gmail.com>

Date: 2025-06-26T06:54:55Z

Lists: pgsql-hackers

Commits

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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 26/6/2025 07:40, Michael Paquier wrote:
> Anyway, it seems to me that we may need to do something here before
> the release.  Note that if the consensus is "you should update your
> module and not rely on the past behavior", I'm OK with that.  I just
> wanted to raise the issue before this goes GA.  And well, I have a
> pretty big pool of users that rely on this module, so..
Thanks for the report and detailed explanation!
Before diving into the pg_hint_plan code, I wonder why you don't have 
similar issues with the remove_useless_joins. We intentionally designed 
SJE coupled with the left-join removal feature to avoid such type of 
complaints:

CREATE TABLE test (x integer PRIMARY KEY);
EXPLAIN (COSTS OFF)
SELECT t1.* FROM test t1 LEFT JOIN test t2 ON (t1.x=t2.x);

/*
      QUERY PLAN
---------------------
  Seq Scan on test t1
(1 row)
*/

It seems that this join removal is also beyond the pg_hint_plan control ...

-- 
regards, Andrei Lepikhov