Logical Data Storage
Oracle tablespace data is stored in segments, and within segments in extents. Extents are made up of blocks, which form the basic unit of access to and from the SGA and disks.
Segments
Oracle uses four types of segments: data segments, index segments, temporary segments, and rollback segments. A tablespace can hold one or more segments, including segments of all four types and multiple segments of one type, and each segment is wholly contained within a single tablespace. When a segment is dropped, free space becomes available within the tablespace (possibly also leading to fragmentation of data within the tablespace).
Data segments store tables and table data. Clusters are a special kind of data segment where rows are stored ...
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