2016-12-05 16:23 GMT+01:00 Stephen Frost <sfr...@snowman.net>: > Pavel, > > * Pavel Stehule (pavel.steh...@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 I don't defend a design of the application - it is exactly wrong. But sometimes, it can be generated by some tool or it can be a human error. And bad performance can be a big problems on systems, where you cannot to deploy fix simply. It is hard to say what should be good design - because these queries are slow, I know so these queries are wrong, but these queries does significant IO utilization - and I cannot to fix the application, because I am not a author and the fix will not be available in next week. Regards Pavel > > > Same issue is a expression a = a .. can be replaced by true > > a = a can't be replaced unless you know that 'a' can't be NULL. > > In short, fix the application. > > Thanks! > > Stephen >
