The 8.1 Release Notes have the following item:
* Allow non-consecutive index columns to be used in a multi-column index
So isn't the following paragraph from "Multicolumn Indexes" out of
date?
The query planner can use a multicolumn index for queries that
involve the leftmost column in the index definition plus any
number of columns listed to the right of it, without a gap. For
example, an index on (a, b, c) can be used in queries involving
all of a, b, and c, or in queries involving both a and b, or in
queries involving only a, but not in other combinations. (In a
query involving a and c the planner could choose to use the index
for a, while treating c like an ordinary unindexed column.)
http://developer.postgresql.org/docs/postgres/indexes-multicolumn.html
--
Michael Fuhr
