On Mon, Dec 5, 2016 at 3:24 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > Pavel Stehule <pavel.steh...@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? a <> a : (a IS NULL) AND NULL a = a : (a IS NOT NULL) OR NULL Pantelis Theodosiou
