Indexes

Hello,

is there any way to speedup queryes like

select ... from [table a] where [int field b] = [number] order by [datetime 
field c] desc limit [number];

with index on field b I get something, but pgsql afterwards resolves to 
sorting manually. 

partial indices with index on field c where field b = number works, but are 
dirty and are becoming annoying as number of different values of field b 
grows.

Any suggestions?

Primoz