Database segments

Segments are named subsets of the database devices on which a particular database is stored. A segment is a label that points to one or more database devices. Within a particular database, you can define segments for the database devices already allocated to that database.

Each database can contain up to 32 segments. Every database contains segments called system, logsegment, and default. You can define additional segments, as needed.

Defining segments for a database lets you control placement of the objects that consume the most storage—tables and indexes. This gives you several performance and control advantages:

By placing large tables on segments that span multiple physical devices, you can increase I/O throughput.
By separating tables and their nonclustered indexes on different physical devices, you can also increase I/O throughput.
By placing a table on a segment of a specific size, you can control space usage, since a table cannot grow larger than its segment allocation.

To monitor the use of space on a segment so that you can take action before a segment becomes full, you can define threshold values. Thresholds allow you to automatically trigger actions such as notification or dumping a transaction log when a segment is filled to the level of the threshold.