* Pavel Stehule (pavel.stehule@gmail.com) wrote: > 2016-12-05 16:23 GMT+01:00 Stephen Frost <sfrost@snowman.net>: > > * Pavel Stehule (pavel.stehule@gmail.com) wrote: > > > 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. > > > > a <> a could go to NULL. Obviously, that'll be false for such a simple > > case, but it might not work out that way in a more complicated WHERE > > clause. > > > > it should be false everywhere
No, it's NULL, not false, if 'a' is NULL:
=# SELECT 1 WHERE (NULL <> NULL) IS NULL; ?column? ---------- 1 (1 row)
=*# SELECT 1 WHERE (FALSE) IS NULL; ?column? ---------- (0 rows)
You can not make the assumption that 'a <> a' is always false.
ok - when the expression is tested on NULL, then a <> a should not be reduced. But when there are not this test, then I can replace this expression by false.