Re: Removing unneeded self joins

Andrei Lepikhov <lepihov@gmail.com>

From: Andrei Lepikhov <lepihov@gmail.com>

To: Michael Paquier <michael@paquier.xyz>

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: 2025-06-27T06:46:29Z

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 27/6/2025 02:26, Michael Paquier wrote:
> 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.
Parse tree transformation (pull-ups, flattenings) adds joins, constant 
clause evaluation or partition pruning may smash whole subtrees - the 
optimisation process is quite unstable from the query structure's point 
of view.
If I understand correctly, pg_hint_plan needs to differentiate 'never 
existed' relations and removed ones. In that sense, up to v.15, Postgres 
saved RelOptInfo after removing the join (see e9a20e4).

-- 
regards, Andrei Lepikhov