Using indexes for ORDER BY and PARTITION BY clause in windowing functions

Hi,

I was dealing with windowing function recently. I feel they are pretty useful and quite handy in lot of operations. 

I am not sure why but my PostgreSQL does not seem to be using indexes for ORDER BY clause or PARTITION BY CLAUSE which I use with windowing function. I have tried ORDER BY and GROUP BY clauses in a normal sql statement and they seem to use indexes nicely.

Is this being already considered for development?