Why can't I use windowing functions over ordered aggregates?

Hackers,

So, I can create a custom aggregate "first" and do this:

SELECT first(val order by ts desc) ...

And I can do this:

SELECT first_value(val) OVER (order by ts desc)

... but I can't do this:

SELECT first_value(val order by ts desc)

... even though under the hood, it's the exact same operation.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com