Pavel Stehule <pavel.stehule@gmail.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?