Re: missing optimization - column <> column

Actually there are lots of things that can be done with this sort of theorem proving.
And NULL is a plenty good answer for a filter, just not for a check constraint.
Amongst them INSERT through UNION ALL for symmetric views which can be handy for FDW partitioned tables.

One such implementation an be found here:
https://www.google.com/patents/US6728952 (apparently expired)

Cheers
Serge

Salesforce.com

> On Dec 5, 2016, at 7:28 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>
>
>
> 2016-12-05 16:24 GMT+01:00 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us <mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us>>:
> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com <mailto:pavel(dot)stehule(at)gmail(dot)com>> writes:
> > I found some crazy queries in one customer application. These queries are
> > stupid, but it was surprise for me so there are not some simple optimization
>
> > create table foo(a int);
> > insert into foo select generate_series(1,100000);
> > analyze foo;
> > explain select * from foo where a <> a;
>
> > It does full scan of foo, although it should be replaced by false in
> > planner time.
>
> > Same issue is a expression a = a .. can be replaced by true
>
> Wrong; those expressions yield NULL for NULL input. You could perhaps
> optimize them slightly into some form of is-null test, but it hardly
> seems worth the planner cycles to check for.
>
> understand
>
>
> If you write something like "1 <> 1", it will be folded.
>
> it works, but a <> a not
>
> Regards
>
> Pavel
>
> regards, tom lane
>