Re: Removing unneeded self joins

On 5/8/21 2:00 AM, Hywel Carver wrote:
> On Fri, May 7, 2021 at 8:23 AM Andrey Lepikhov
> <a(dot)lepikhov(at)postgrespro(dot)ru <mailto:a(dot)lepikhov(at)postgrespro(dot)ru>> wrote:
> Here I didn't work on 'unnecessary IS NOT NULL filter'.
>
> I've tested the new patch, and it is giving the same improved behaviour
> as the old patch.
Thank you for this efforts!

I cleaned the code of previous version, improved regression tests and
rebased on current master.

Also, I see that we could do additional optimizations for an
EC-generated selfjoin clause (See equivclass.patch for necessary
changes). Example:
explain (costs off)
select * from sj t1, sj t2 where t1.a = t1.b and t1.b = t2.b and t2.b =
t2.a;
QUERY PLAN
-----------------------------------------------------
Seq Scan on sj t2
Filter: ((a IS NOT NULL) AND (b = a) AND (a = b))
(2 rows)

But I'm not sure that this patch need to be a part of the self-join
removal feature because of code complexity.

--
regards,
Andrey Lepikhov
Postgres Professional