Re: missing optimization - column <> column

On Mon, Dec 5, 2016 at 3:24 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Pavel Stehule <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.
>
> If you write something like "1 <> 1", it will be folded.
>
> regards, tom lane
>
>
>

Would it be worth replacing the condition with the equivalent?
I mean would that help optimizing better some queries when it knows that a
is (not) nullable or when "a" is more complicated expression?

a <> a : (a IS NULL) AND NULL
a = a : (a IS NOT NULL) OR NULL

Pantelis Theodosiou