Re: Removing unneeded self joins

Michael Paquier <michael@paquier.xyz>

From: Michael Paquier <michael@paquier.xyz>

To: Andrei Lepikhov <lepihov@gmail.com>

Cc: Alexander Korotkov <aekorotkov@gmail.com>, 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: 2025-06-27T00:26:31Z

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 Thu, Jun 26, 2025 at 08:54:55AM +0200, Andrei Lepikhov wrote:
> 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 ...

Yeah, remove_useless_joins() has been around since the 9.0 ages as far
as I know, and it's not bothered the module much in the tests because
we have unlikely relied on it.

For now the trick I am going to rely on is just disable
enable_self_join_elimination in the test paths where I rely on the 
same relation and self joins to keep the plans of the regression tests
stable.  This stuff could create separate relations, but then with
back-patching in mind that's going to be just extra conflict noise,
which is always annoying when dealing with plan outputs.  We've done
that in the past with max_parallel_workers_per_gather or jit, to keep
the expected plans stable.

The point regarding the search join hook may stand, though.  Perhaps
somebody should check if we're still OK with this change in the
context of the self-join work.  I tend to think that we are and I
agree that removing the joins when calling the hook can show benefits,
but it may be surprising and users tend to be very noisy with plan
stability, particularly if some of the FROM aliases get silenced by
the backend without the module knowing about that.  At least there is
the trick with SET enable_self_join_elimination available as a last
resort method.
--
Michael