CREATE ROLLBACK SEGMENT

Purpose

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

The information in this section assumes that your database is running in rollback undo mode (the UNDO_MANAGEMENT initialization parameter is set to MANUAL or not set at all).

If your database is running in Automatic Undo Management mode (the UNDO_MANAGEMENT initialization parameter is set to AUTO), then user-created rollback segments are irrelevant. In this case, Oracle returns an error in response to any CREATE ROLLBACK SEGMENT or ALTER ROLLBACK SEGMENT statement. To suppress these errors, set the UNDO_SUPPRESS_ERRORS parameter to TRUE.

Further, if your database has a locally managed SYSTEM tablespace, then you cannot create rollback segments in any dictionary-managed tablespace. Instead, you must

• Use the Automatic Undo Management feature, which uses undo tablespaces instead of rollback segments to hold undo data, or
• Create locally managed tablespaces to hold the rollback segments.

Oracle Corporation recommends that you use Automatic Undo Management.

Notes: A tablespace can have multiple rollback segments. Generally, multiple rollback segments improve performance. The tablespace must be online for you to add a rollback segment to it. When you create a rollback segment, it is initially offline. To make it available for transactions by your Oracle instance, bring it online using the ALTER ROLLBACK SEGMENT statement. To bring it online automatically whenever you start up the database, add the segment's name to the value of the ROLLBACK_SEGMENTS initialization parameter.

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

• If you are running the database in rollback undo mode, at least one rollback segment (other than the SYSTEM rollback segment) must be online.
• If you are running the database in Automatic Undo Management mode, at least one UNDO tablespace must be online.

  See Also: ALTER ROLLBACK SEGMENT for information on altering a rollback segment DROP ROLLBACK SEGMENT for information on removing a rollback segment Oracle9i Database Reference for information on the UNDO_MANAGEMENT and UNDO_SUPPRESS_ERRORS parameters Oracle9i Database Administrator's Guide for information on Automatic Undo Management mode

Prerequisites

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

Syntax

create_rollback_segment::=

Text description of create_rollback_segment

(storage_clause::=)

Keyword and Parameters

PUBLIC

Specify PUBLIC to indicate that the rollback segment is public and is available to any instance. If you omit this clause, 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, Oracle creates the rollback segment in the SYSTEM tablespace.

Note: Oracle must access rollback segments frequently. Therefore, Oracle Corporation strongly recommends that you do not create rollback segments in the SYSTEM tablespace, either explicitly or implicitly (by omitting this clause). In addition, to avoid high contention for the tablespace containing the rollback segment, it should not contain other objects such as tables and indexes, and it should require minimal extent allocation and deallocation. To achieve these goals, create rollback segments in locally managed tablespaces with autoallocation disabled--that is, in tablespaces created with the EXTENT MANAGEMENT LOCAL clause with the UNIFORM setting. (The AUTOALLOCATE setting is not supported.)

See Also: CREATE TABLESPACE Oracle9i Database Administrator's Guide for more information on creating rollback segments and making them available

storage_clause

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

Notes: 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.

See Also: storage_clause

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
     NEXT 10K
     MAXEXTENTS UNLIMITED);