From : http://download-west.oracle.com/docs...tsref.htm#5156


"This hint can optionally specify one or more indexes:

If this hint specifies a single available index, then the optimizer performs a scan on this index. The optimizer does not consider a full table scan or a scan on another index on the table.
If this hint specifies a list of available indexes, then the optimizer considers the cost of a scan on each index in the list and then performs the index scan with the lowest cost. The optimizer can also choose to scan multiple indexes from this list and merge the results, if such an access path has the lowest cost. The optimizer does not consider a full table scan or a scan on an index not listed in the hint.
If this hint specifies no indexes, then the optimizer considers the cost of a scan on each available index on the table and then performs the index scan with the lowest cost. The optimizer can also choose to scan multiple indexes and merge the results, if such an access path has the lowest cost. The optimizer does not consider a full table scan. "