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Tablespaces allocate space in extents. Tablespaces can use two different methods to
keep track of their free and used space:
■ Locally managed tablespaces: Extent management by the tablespace
■ Dictionary managed tablespaces: Extent management by the data dictionary

A tablespace that manages its own extents maintains a bitmap in each datafile to keep
track of the free or used status of blocks in that datafile. Each bit in the bitmap
corresponds to a block or a group of blocks. When an extent is allocated or freed for
reuse, Oracle changes the bitmap values to show the new status of the blocks. These
changes do not generate rollback information because they do not update tables in the
data dictionary (except for special cases such as tablespace quota information).
Locally managed tablespaces have the following advantages over dictionary managed
tablespaces:
■ Local management of extents automatically tracks adjacent free space, eliminating
the need to coalesce free extents.
■ Local management of extents avoids recursive space management operations.
Such recursive operations can occur in dictionary managed tablespaces if
consuming or releasing space in an extent results in another operation that
consumes or releases space in a data dictionary table or rollback segment.