Reclaiming space in an empty extent with ALTER INDEX

If the table with the empty extents includes an index, you can run the ALTER INDEX statement with the TO CLUSTER clause. Clustering an index rebuilds the table in a different location within the dbspace.

When you run the ALTER INDEX statement with the TO CLUSTER clause, all of the extents associated with the previous version of the table are released. Also, the newly built version of the table has no empty extents.