Re: Removing unneeded self joins

Andrei Lepikhov <a.lepikhov@postgrespro.ru>

From: "Andrey V. Lepikhov" <a.lepikhov@postgrespro.ru>

To: Hywel Carver <hywel@skillerwhale.com>, pgsql-hackers <pgsql-hackers@lists.postgresql.org>

Date: 2021-05-27T06:51:23Z

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

Attachments

equivclass.patch (text/x-patch) patch
v27-0001-Remove-self-joins.patch (text/x-patch)

On 5/8/21 2:00 AM, Hywel Carver wrote:
> On Fri, May 7, 2021 at 8:23 AM Andrey Lepikhov 
> <a.lepikhov@postgrespro.ru <mailto:a.lepikhov@postgrespro.ru>> wrote:
>     Here I didn't work on 'unnecessary IS NOT NULL filter'.
> 
> I've tested the new patch, and it is giving the same improved behaviour 
> as the old patch.
Thank you for this efforts!

I cleaned the code of previous version, improved regression tests and 
rebased on current master.

Also, I see that we could do additional optimizations for an 
EC-generated selfjoin clause (See equivclass.patch for necessary 
changes). Example:
explain (costs off)
select * from sj t1, sj t2 where t1.a = t1.b and t1.b = t2.b and t2.b = 
t2.a;
                      QUERY PLAN
-----------------------------------------------------
  Seq Scan on sj t2
    Filter: ((a IS NOT NULL) AND (b = a) AND (a = b))
(2 rows)

But I'm not sure that this patch need to be a part of the self-join 
removal feature because of code complexity.

-- 
regards,
Andrey Lepikhov
Postgres Professional