The Oracle Database provides a Segment Advisor that helps you determine whether
an object has space available for reclamation based on the level of space fragmentation
within the object.

In general, the extents of a segment do not return to the tablespace until you drop the
schema object whose data is stored in the segment (using a DROP TABLE or DROP
CLUSTER statement). Exceptions to this include the following:
■ The owner of a table or cluster, or a user with the DELETE ANY privilege, can
truncate the table or cluster with a TRUNCATE...DROP STORAGE statement.
■ A database administrator (DBA) can deallocate unused extents using the following
SQL syntax:
ALTER TABLE table_name DEALLOCATE UNUSED;

■ Periodically, Oracle deallocates one or more extents of a rollback segment if it has
the OPTIMAL size specified.
When extents are freed, Oracle modifies the bitmap in the datafile (for locally managed tablespaces) or updates the data dictionary (for dictionary managed tablespaces) to reflect the regained extents as available space. Any data in the blocks of
freed extents becomes inaccessible.

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When Extents Are Deallocated (15)

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