Re: Removing unneeded self joins

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