Re: Removing unneeded self joins

Alexander Korotkov <aekorotkov@gmail.com>

From: Alexander Korotkov <aekorotkov@gmail.com>

To: Alexander Lakhin <exclusion@gmail.com>

Cc: Andrei Lepikhov <a.lepikhov@postgrespro.ru>, "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Michał Kłeczek <michal@kleczek.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>

Date: 2024-05-01T11:59:58Z

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

sje_tablesample.patch (application/octet-stream) patch

On Wed, May 1, 2024 at 2:00 PM Alexander Lakhin <exclusion@gmail.com> wrote:
> 30.04.2024 13:20, Alexander Korotkov wrote:
> > On Tue, Apr 30, 2024 at 9:00 AM Alexander Lakhin <exclusion@gmail.com> wrote:
> >> I've discovered another failure, introduced by d3d55ce57.
> >> Please try the following:
> >> CREATE TABLE t (a int unique, b float);
> >> SELECT * FROM t NATURAL JOIN LATERAL
> >>    (SELECT * FROM t t2 TABLESAMPLE SYSTEM (t.b)) t2;
> > I think we should just forbid SJE in case when relations to be merged
> > have cross-references with lateral vars.  The draft patch for this is
> > attached.  I'd like to ask Alexander to test it, Richard and Andrei to
> > review it.  Thank you!
>
> Beside LATERAL vars, it seems that SJR doesn't play well with TABLESAMPLE
> in general. For instance:
> CREATE TABLE t (a int unique);
> INSERT INTO t SELECT * FROM generate_series (1,100);
>
> SELECT COUNT(*) FROM (SELECT * FROM t TABLESAMPLE BERNOULLI(1)) t1
>   NATURAL JOIN (SELECT * FROM t TABLESAMPLE BERNOULLI(100)) t2;
> returned 100, 100, 100 for me, though with enable_self_join_removal = off,
> I got 4, 0, 1...

Right, thank you for reporting this.

BTW, I found another case where my previous fix doesn't work.

SELECT * FROM t NATURAL JOIN LATERAL (SELECT * FROM t t2 TABLESAMPLE
SYSTEM (t.b) NATURAL JOIN LATERAL(SELECT * FROM t t3 TABLESAMPLE
SYSTEM (t2.b)) t3) t2;

I think we probably could forbid SJE for the tables with TABLESAMPLE
altogether.  Please, check the attached patch.

------
Regards,
Alexander Korotkov