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.lepikhov@postgrespro.ru <mailto:a.lepikhov@postgrespro.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