CREATE ROLLBACK SEGMENT

Purpose

Use the CREATE ROLLBACK SEGMENT statement to create a rollback segment, which is an object that Oracle Database uses to store data necessary to reverse, or undo, changes made by transactions.

The information in this section assumes that your database is not running in automatic undo mode (the UNDO_MANAGEMENT initialization parameter is set to MANUAL or not set at all). If your database is running in automatic undo mode (the UNDO_MANAGEMENT initialization parameter is set to AUTO, which is the default), then user-created rollback segments are irrelevant.

Further, if your database has a locally managed SYSTEM tablespace, then you cannot create rollback segments in any dictionary-managed tablespace. Instead, you must either use the automatic undo management feature or create locally managed tablespaces to hold the rollback segments.

To use objects in a tablespace other than the SYSTEM tablespace:

• If you are using rollback segments for undo, then at least one rollback segment (other than the SYSTEM rollback segment) must be online.

• If you are running the database in automatic undo mode, then at least one UNDO tablespace must be online.

Prerequisites

To create a rollback segment, you must have the CREATE ROLLBACK SEGMENT system privilege.

Syntax

create_rollback_segment::=

(storage_clause)

Semantics

PUBLIC

Specify PUBLIC to indicate that the rollback segment is public and is available to any instance. If you omit this clause, then the rollback segment is private and is available only to the instance naming it in its initialization parameter ROLLBACK_SEGMENTS.

rollback_segment

Specify the name of the rollback segment to be created.

TABLESPACE

Use the TABLESPACE clause to identify the tablespace in which the rollback segment is created. If you omit this clause, then the database creates the rollback segment in the SYSTEM tablespace.

storage_clause

The storage_clause lets you specify storage characteristics for the rollback segment.

• The OPTIMAL parameter of the storage_clause is of particular interest, because it applies only to rollback segments.

• You cannot specify the PCTINCREASE parameter of the storage_clause with CREATE ROLLBACK SEGMENT.

Examples

Creating a Rollback Segment: Example The following statement creates a rollback segment with default storage values in an appropriately configured tablespace:

CREATE TABLESPACE rbs_ts
   DATAFILE 'rbs01.dbf' SIZE 10M
   EXTENT MANAGEMENT LOCAL UNIFORM SIZE 100K;

/* This example and the next will fail if your database is in 
   automatic undo mode.
*/
CREATE ROLLBACK SEGMENT rbs_one
   TABLESPACE rbs_ts;

The preceding statement is equivalent to the following:

CREATE ROLLBACK SEGMENT rbs_one
   TABLESPACE rbs_ts
   STORAGE
   ( INITIAL 10K );