using a btree index in order by clause?

Hi,

I'm wondering if we can use btree index to sort the results in a
certain condition. The idea is, if the order-items in the order by
clause have a btree index, then why we need to sort them again?

I'm starting to look at the executor code. However this kind of
"optimization" might be better to be done in the optimizer, I don't
know.

Suggestion?
--
Tatsuo Ishii
t-ishii@sra.co.jp