introduction of WIP window function patch

Hi,

As I proposed a month before, I am working on window function.
Although this work is at quite early step, I would like to introduce
it since a part of it have been finished. If you can afford and are
interested in it, please review the document and patch, or compile the
applied source to execute an attached sample SQL.

http://umitanuki.net/pgsql/wfv01/design.html

Currently, only aggregation over window does work. I am planning to
work for the combination of window and normal aggregation from now on,
which I guess I can manage to do.

The problem is, as written in the "Things to discussed" section of the
document, how you define window functions (e.g. RANK()). My idea is to
treat them as specialized functions such as SET OF functions and mark
it in pg_proc. But this doesn't resolve RANK() boundary problem.

I am so happy with any kind of comments, reviews or critiques.

Regards,

-- 
Hitoshi Harada