Oracle7 Server Concepts
The Sequence Generator
The sequence generator provides a sequential series of numbers. The sequence generator is especially useful in multi-user environments for generating unique sequential numbers without the overhead of disk I/O or transaction locking. Therefore, the sequence generator reduces "serialization" where the statements of two transactions must generate sequential numbers at the same time. By avoiding the serialization that results when multiple users wait for each other to generate and use a sequence number, the sequence generator improves transaction throughput and a user's wait is considerably shorter.
Sequence numbers are Oracle integers defined in the database of up to 38 digits. A sequence definition indicates general information: the name of the sequence, whether it ascends or descends, the interval between numbers, and other information. One important part of a sequence's definition is whether Oracle should cache sets of generated sequence numbers in memory. Oracle stores the definitions of all sequences for a particular database as rows in a single data dictionary table in the SYSTEM tablespace. Therefore, all sequence definitions are always available, because the SYSTEM tablespace is always online.
Sequence numbers are used by SQL statements that reference the sequence. You can issue a statement to generate a new sequence number or use the current sequence number. Once a statement in a user's session generates a sequence number, the particular sequence number is available only to that session; each user that references a sequence has access to its own, current sequence number.
Sequence numbers are generated independently of tables. Therefore, the same sequence generator can be used for one or for multiple tables. Sequence number generation is useful to generate unique primary keys for your data automatically and to coordinate keys across multiple rows or tables. Individual sequence numbers can be skipped if they were generated and used in a transaction that was ultimately rolled back. Applications can make provisions to catch and reuse these sequence numbers, if desired.
For more performance implications when using sequences, see the Oracle7 Server Application Developer's Guide.