These views provide information about internal storage in the database, including datafiles, tablespaces, free extents, used extents, and segments:
Lists all data files in use by the database.
Lists every allocated extent for every segment.
Lists every free extent. With DBA_EXTENTS, should account for all storage in DBA_DATA_FILES.
Lists every extent that is at the start of a block of free extents.
Lists all rollback segments.
Lists all segments.
Lists all tablespaces.
Shows the granted quota and used storage in tablespaces by user.