Extents
Extents are allocated to a segment when that segment is created, extended, or altered. They are deallocated when the segment is dropped, altered, or truncated.
When a tablespace is created, data files associated with that tablespace contain a header, the first block, or set of blocks, in each data file. Whenever a segment gets created, it is allocated space from the tablespace's free extents. The contiguous space being used by a segment is referred to as used extents. If the segment releases space, extents that are released are added to the pool of free extents available to the tablespace to be allocated to any segment associated with that tablespace.
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