Re: Removing unneeded self joins

Alexander Korotkov <aekorotkov@gmail.com>

From: Alexander Korotkov <aekorotkov@gmail.com>

To: Andrei Lepikhov <a.lepikhov@postgrespro.ru>

Cc: Alexander Lakhin <exclusion@gmail.com>, Richard Guo <guofenglinux@gmail.com>, "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Michał Kłeczek <michal@kleczek.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2024-05-02T10:07:54Z

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, May 2, 2024 at 12:45 PM Andrei Lepikhov
<a.lepikhov@postgrespro.ru> wrote:
>
> On 5/1/24 18:59, Alexander Korotkov wrote:
> > I think we probably could forbid SJE for the tables with TABLESAMPLE
> > altogether.  Please, check the attached patch.
> Your patch looks good to me. I added some comments and test case into
> the join.sql.

Thank you

> One question for me is: Do we anticipate other lateral self-references
> except the TABLESAMPLE case? Looking into the extract_lateral_references
> implementation, I see the only RTE_SUBQUERY case to be afraid of. But we
> pull up subqueries before extracting lateral references. So, if we have
> a reference to a subquery, it means we will not flatten this subquery
> and don't execute SJE. Do we need more code, as you have written in the
> first patch?

I think my first patch was crap anyway.  Your explanation seems
reasonable to me.  I'm not sure this requires any more code.  Probably
it would be enough to add more comments about this.

------
Regards,
Alexander Korotkov