gaurav_varma

 Profile Answers by gaurav_varma 

• Jan 10th, 2007

 

A segment is a set of extents that contains all the data for a specific logical storage structure within a tablespace. For example, for each table, Oracle allocates one or more extents to form that table's data segment; for each index, Oracle allocates one or more extents to form its index segment.

Oracle databases use four types of segments, these are:

Data Segments

Index Segments

Temporary Segments

Rolback Segments

Data Segments: A single data segment in an Oracle database holds all of the data for one of the following:

• a table that is not partitioned or clustered

• a partition of a partitioned table

• a cluster of tables

Oracle creates this data segment when you create the table or cluster with the CREATE command.

The storage parameters for a table or cluster determine how its data segment's extents are allocated. You can set these storage parameters directly with the appropriate CREATE or ALTER command. These storage parameters affect the efficiency of data retrieval and storage for the data segment associated with the object.

Index Segments: Every nonpartitioned index in an Oracle database has a single index segment to hold all of its data. For a partitioned index, every partition has a single index segment to hold its data.

Oracle creates the index segment for an index or an index partition when you issue the CREATE INDEX command. In this command, you can specify storage parameters for the extents of the index segment and a tablespace in which to create the index segment. (The segments of a table and an index associated with it do not have to occupy the same tablespace.) Setting the storage parameters directly affects the efficiency of data retrieval and storage.

Temporary Segments: When processing queries, Oracle often requires temporary workspace for intermediate stages of SQL statement parsing and execution. Oracle automatically allocates this disk space called a temporary segment. Typically, Oracle requires a temporary segment as a work area for sorting. Oracle does not create a segment if the sorting operation can be done in memory or if Oracle finds some other way to perform the operation using indexes

Rollback Segments: Each database contains one or more rollback segments. A rollback segment records the old values of data that was changed by each transaction (whether or not committed). Rollback segments are used to provide read consistency, to roll back transactions, and to recover the database