BUG #6440: Window function in WHERE evaluated after agregate

The following bug has been logged on the website:

Bug reference: 6440
Logged by: Matyas Novak
Email address: logik(at)centrum(dot)cz
PostgreSQL version: 9.1.1
Operating system: Linux
Description:

Window function in WHERE clause - as all other functions used in where -
should be evaluated before agregating takes place. But it seems that
postgresql try evaluate its after agregating, as it'd be in HAVING clause.

E.g. in folowing example I'l try to sum the best results of given persons
from all dispciplines - so I agregate over persons and best result in each
discipline wann get using window function.

(I'm aware that it can be done by various subselects, or that there may be
better to use different window function or nested agregates, but it's
simplified example from a bit complex example.)

Test example:

create table results
(
id serial not null primary key,
person integer,
discipline integer,
result integer
);

select person, sum(result) from
results
where
row_number() over (partition by person, discipline order by result desc) =
1
group by
person

Error:
ERROR: column "results.result" must appear in the GROUP BY clause or be
used in an aggregate function
LINE 19: ...() over (partition by person, discipline order by result des...