Oracle logical database structure: Segment

A segment is the logical container for

• a nonpartitioned schema objects
• a partition of partitioned objects
• a cluster of tables

A segment belongs to exactly one tablespace and consists of of one or more extents.

The data of a segment is stored in at least one data file, but can easily span multiple data files.

In the case of a hybrid partitioned table however, some data is stored via (ordinary) segments in data files and other data is stored in external data sources.

In the data dictionary, storage allocation for segments is recorded in dba_segments.

Types of segments:

• User segments
• Undo segments
• Temporary segments

A segment in manual segment space management (MSSM) maintains a free list which keeps track of free blocks below a segment's high water mark.

Some statistical numbers about segment usage is found in v$segment_statistics.