calculated expressions and index use

Hi all,

I use simple sql statement like this

select * from foo
where created_at >= 'now'::timestamp - '1 hour'::interval;

My table is indexed on created_at field. The query above doesn't use it, 
but if I use

select * from foo where created_at >= 'now'

the index is used.

It looks like if the engine finds some expression to be evaluated it 
gets rid of any index use (due the possible dependency?).

Is it possible to do something to 'precompute' some value in the query 
condition, so planner understand is as a constant value?

(... and I've got some reason NOT to use a parameter in my Perl DBI code...)

Maybe my conclusions are wrong, of course.

Thank you very much in advance

Pavel

-- 
Pavel Hlavnicka
Ginger Alliance
www.gingerall.com