Data blocks are used by Oracle in all I/O operations. A segment is composed of one or more extents, but all the data in a table or an index must be contained within a single segment.
Database objects, such as tables and indexes, are held in specific segments. You do not specifically create segments--they are automatically created to support different types of storage.
Types of segments
There are four types of segments used by Oracle:
Data segment: A data segment is created each time you create a table. The number and size of extents
for a data segment is specified in the CREATE TABLE statement.
Index segment: An index segment is created each time you create an index. The number and size of
extents for an index segment is specified in the CREATE INDEX statement. (Ques.Use index)
Temporary segment: A temporary segment may be necessary to provide temporary storage for database
operations, such as sorting. Temporary segments are allocated for the temporary
tablespace of users who require the additional space for temporary storage.
Rollback segment: A rollback segment contains information needed by your Oracle database to roll back
transactions, if necessary. Rollback segments are allocated to the database and cannot
be directly accessed by users or database administrators.
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