segment (thing)

In the Oracle RDBMS, a segment is a logical data structure constructed out of extents. The four segment types are:

• Data segments are associated with (non-clustered) tables and contains the data stored within that table (or within a table partition)
• Index segments contain the data that the index is made up of
• Rollback segments contain undo information that is used in many very important ways, such as to ensure read-consistant views of data (for example, to implement an ACID transaction)
• Temporary segments store temporary data, to sort output, for example.

Segments are the largest of three data allocation structures in Oracle. Segments are constructed from extents, and extents are constructed from data blocks.