Re: Removing unneeded self joins

Richard Guo <guofenglinux@gmail.com>

From: Richard Guo <guofenglinux@gmail.com>

To: Alexander Korotkov <aekorotkov@gmail.com>

Cc: Ranier Vilela <ranier.vf@gmail.com>, Alena Rybakina <a.rybakina@postgrespro.ru>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Tom Lane <tgl@sss.pgh.pa.us>, Andrei Lepikhov <lepihov@gmail.com>, 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: 2025-04-04T02:53:54Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

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 Fri, Apr 4, 2025 at 1:02 AM Alexander Korotkov <aekorotkov@gmail.com> wrote:
> I've got an off-list bug report from Alexander Lakhin involving a
> placeholder variable.  Alena and Andrei proposed a fix.  It is fairly
> simple: we just shouldn't remove PHVs during self-join elimination, as
> they might still be referenced from other parts of a query.  The patch
> is attached.  I'm going to fix this if no objections.

Hmm, I'm not sure about the fix.  It seems to me that it simply
prevents removing any PHVs in the self-join removal case.  My concern
is that this might result in PHVs that could actually be removed not
being removed in many cases.

Besides, there's the specific comment above this code explaining the
logic behind the removal of PHVs.  Shouldn't that comment be updated
to reflect the changes?

Thanks
Richard