index on more-than-one columns?

Hi all,

Is there any advantage to use index on more columns against to use
more separate indexes, e.g. creating index on t1 (a,b) instead of
index on t1 (a) plus index on t1 (b)? Let's suppose that the queries
include all sorts of combinations.

Thx:
Circum
__  @
/  \    _   _                                           Engard Ferenc
l    | ( \ /  | | (\/)                      mailto:s-fery@kkt.sote.hu
\__/ | |   \_ \_/ I  I                    http://pons.sote.hu/~s-fery