Re: Removing unneeded self joins

Alexander Korotkov <aekorotkov@gmail.com>

From: Alexander Korotkov <aekorotkov@gmail.com>

To: Michael Paquier <michael@paquier.xyz>

Cc: Andrei Lepikhov <lepihov@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: 2025-07-15T22:29:35Z

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 Wed, Jul 16, 2025 at 1:16 AM Michael Paquier <michael@paquier.xyz> wrote:
> On Wed, Jul 16, 2025 at 12:38:58AM +0300, Alexander Korotkov wrote:
> > I recently got notification this is in Open Items.
> > https://wiki.postgresql.org/wiki/PostgreSQL_18_Open_Items
> > What is your opinion on this: do we need additional hook, fair to
> > leave this "as is" or another option?
>
> I'm OK with the statu-quo on my side when it comes to plan hinting.
> With the GUC workaround, it's still possible to get through so it is
> not like we don't have any options.  The point about other extensions
> still stands, I think, but perhaps we are OK even on these fronts as
> the join search hook is far from being the most popular one AFAIK.
>
> So dropping the item and do nothing is a fine answer.

OK, thank you for your feedback.  I've searched GitHub for other users
of join_search_hook.  A couple of interesting use cases I found are
the following.
https://github.com/wulczer/saio
https://github.com/ashenBlade/pg_dphyp
But they provide alternative join optimization algorithms.  So, in
spite of pg_hint_plan they are probably OK to optimize whatever joins
are left.  Thus, it's not yet clear whether this issue affects anybody
else.  I've moved this item into the "non-bugs" list, but I'm OK to
re-open it if there are other affected extensions.

------
Regards,
Alexander Korotkov
Supabase