Vlad Arkhipov <arhipov(at)dc(dot)baikal(dot)ru> writes:
> I wonder why Postgres does not use index in the query below?
Because it's useless: you'd still have to do a sort, and an indexscan
is going to be a slower source of data for the sort than a seqscan.
There's been some experimentation of late with a "partial sort" capability
that could take advantage of partially-ordered input, which might make
this kind of thing interesting after all. But it's not committed and
might never be: it's far from clear that it'd be a win in many cases.
regards, tom lane