Actually there are lots of things that can be done with this sort of theorem
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)
> On Dec 5, 2016, at 7:28 AM, Pavel Stehule <pavel.steh...@gmail.com> wrote:
> 2016-12-05 16:24 GMT+01:00 Tom Lane <t...@sss.pgh.pa.us
> Pavel Stehule <pavel.steh...@gmail.com <mailto:pavel.steh...@gmail.com>>
> > 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.
> If you write something like "1 <> 1", it will be folded.
> it works, but a <> a not
> regards, tom lane