Separating tables and indexes

Use segments to isolate tables on one set of disks and nonclustered indexes on another set of disks. You cannot place a clustered index on a separate segment than its data pages. When you create a clustered index, using the on segment_name clause, the entire table is moved to the specified segment, and the clustered index tree is built there.

You can improve performance by placing nonclustered indexes on a separate segment.