Re: Removing unneeded self joins - Mailing list pgsql-hackers

On Tue, Jan 9, 2024 at 6:00 AM Alexander Lakhin <exclusion@gmail.com> wrote:
> 09.01.2024 01:09, Alexander Korotkov wrote:
> > Fixed in 30b4955a46.
>
> Thank you for fixing that!
>
> I've found another anomaly coined with d3d55ce57. This query:
> CREATE TABLE t(a int PRIMARY KEY, b int);
> INSERT INTO t VALUES  (1, 1), (2, 1);
>
> WITH t1 AS (SELECT * FROM t)
> UPDATE t SET b = t1.b + 1 FROM t1
> WHERE t.a = t1.a RETURNING t.a, t1.b;
>
> gives "ERROR:  variable not found in subplan target lists" on d3d55ce57, but
> starting from a7928a57b it gives an incorrect result:
>   a | b
> ---+---
>   1 | 2
>   2 | 2
> (2 rows)

I see.  It seems to be not safe to apply SJE to the modify table
target relation because it could use a different snapshot for the
RETURNING clause.  I think we should just forbid SJE to involve the
modify table target relation.  I'm planning to fix this later today.

------
Regards,
Alexander Korotkov