Segment

A segment is a database object that has space allocated to it - for example, a table, index, materialized view, etc. A segment consists of one or more extents allocated within a tablespace.

Segment types

Some of the available segment types are:

• CLUSTER
• INDEX
• INDEX PARTITION
• LOB PARTITION
• LOBINDEX
• LOBSEGMENT
• NESTED TABLE
• ROLLBACK
• TABLE
• TABLE PARTITION
• TYPE2 UNDO

Segment headers

The segment header is stored in the first block of the first extent. It contains:

• an extents table
• freelist descriptors
• high water mark of the segment

List segments

Catalog views: USER_SEGMENTS and DBA_SEGMENTS show segment information. Note there is no ALL_SEGMENTS view. Example:

SELECT segment_name, segment_type, tablespace_name, extents
  FROM user_segments;

SEGMENT_NAME SEGMENT_TYPE TABLESPACE_NAME    EXTENTS
------------ ------------ --------------- ----------
DEPT         TABLE        USERS                    1
PK_DEPT      INDEX        USERS                    1
EMP          TABLE        USERS                    1
PK_EMP       INDEX        USERS                    1
BONUS        TABLE        USERS                    1
SALGRADE     TABLE        USERS                    1

6 rows selected.

Also see

• Extent - contiguous set of data blocks allocated to a segment

Glossary of Terms
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