Re: Removing unneeded self joins

Andrei Lepikhov <lepihov@gmail.com>

From: Andrei Lepikhov <lepihov@gmail.com>

To: Alexander Korotkov <aekorotkov@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>

Cc: Alena Rybakina <a.rybakina@postgrespro.ru>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Robert Haas <robertmhaas@gmail.com>, Richard Guo <guofenglinux@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: 2025-02-23T22:12:36Z

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 23/2/2025 22:15, Alexander Korotkov wrote:
> There is my attempt to implement this approach.  Getting rid of local
> variable (and computation of the same value other way) required to
> change arguments of remove_rel_from_eclass() as well.  I'm going to
> further polish this tomorrow.
I passed through the patch. It works correctly.

Multiple ifs in a single routine is not ideal. I have thought about it 
already, and it seems the remove_rel_from_query needs refactoring: when 
I first reused it for self-join removal, we didn't have the 'ojrelid' 
machinery, and it was implemented smoothly.
Right now, this code contains multiple places where we need to remove 
the outer join relid and separating the removal code for the baserel and 
outer join may simplify the logic and make it safer.
But the way to do it is not apparent now. May be if we implement a new 
technique of query tree reduction, the approach will become more evident.

-- 
regards, Andrei Lepikhov