Use of data within indexes

Hi,

Just wondering... Is Postgresql able to use data present within indexes 
without looking up the table data?

To be more explicit, let's say I have table with two fields a and b. If I 
have an index on (a,b) and I do a request like "SELECT b FROM table WHERE 
a=x", will Postgresql use only the index, or will it need to also read the 
table page for that (those) row(s)?

There might be a reason why this is not possible (I don't know if the 
indexes have all necessary transaction ID information?) but otherwise this 
could possibly provide an interesting performance gain for some operations, 
in particular with some types of joins. Or maybe it already does it.

Any hint welcome!

Thanks,

Jacques.