This section describes the various steps involved in undo tablespace management and contains the following sections:
Creating an Undo Tablespace
There are two methods of creating an undo tablespace. The first method creates the undo tablespace when the CREATE DATABASE statement is issued. This occurs when you are creating a new database, and the instance is started in automatic undo management mode (UNDO_MANAGEMENT = AUTO). The second method is used with an existing database. It uses the CREATE UNDO TABLESPACE statement.
You cannot create database objects in an undo tablespace. It is reserved for system-managed undo data.
Oracle Database enables you to create a single-file undo tablespace.
Using CREATE DATABASE to Create an Undo Tablespace
You can create a specific undo tablespace using the UNDO TABLESPACE clause of the CREATE DATABASE statement.
The following statement illustrates using the UNDO TABLESPACE clause in a CREATE DATABASE statement. The undo tablespace is named undotbs_01 and one datafile, /u01/oracle/rbdb1/undo0101.dbf, is allocated for it.
If the undo tablespace cannot be created successfully during CREATE DATABASE, the entire CREATE DATABASE operation fails. You must clean up the database files, correct the error and retry the CREATE DATABASE operation.
The CREATE DATABASE statement also lets you create a single-file undo tablespace at database creation.
Using the CREATE UNDO TABLESPACE Statement
The CREATE UNDO TABLESPACE statement is the same as the CREATE TABLESPACE statement, but the UNDO keyword is specified. The database determines most of the attributes of the undo tablespace, but you can specify the DATAFILE clause.
This example creates the undotbs_02 undo tablespace:
You can create more than one undo tablespace, but only one of them can be active at any one time.
Altering an Undo Tablespace
Undo tablespaces are altered using the ALTER TABLESPACE statement. However, since most aspects of undo tablespaces are system managed, you need only be concerned with the following actions:
These are also the only attributes you are permitted to alter.
If an undo tablespace runs out of space, or you want to prevent it from doing so, you can add more files to it or resize existing datafiles.
The following example adds another datafile to undo tablespace undotbs_01:
You can use the ALTER DATABASE ... DATAFILE statement to resize or extend a datafile.
Dropping an Undo Tablespace
Use the DROP TABLESPACE statement to drop an undo tablespace. The following example drops the undo tablespace undotbs_01:DROP TABLESPACE undotbs_01;
An undo tablespace can only be dropped if it is not currently used by any instance. If the undo tablespace contains any outstanding transactions (for example, a transaction died but has not yet been recovered), the DROP TABLESPACE statement fails. However, since DROP TABLESPACE drops an undo tablespace even if it contains unexpired undo information (within retention period), you must be careful not to drop an undo tablespace if undo information is needed by some existing queries.
DROP TABLESPACE for undo tablespaces behaves like DROP TABLESPACE ...INCLUDING CONTENTS. All contents of the undo tablespace are removed.
Switching Undo Tablespaces
You can switch from using one undo tablespace to another. Because the UNDO_TABLESPACE initialization parameter is a dynamic parameter, the ALTER SYSTEM SET statement can be used to assign a new undo tablespace. The following statement switches to a new undo tablespace:ALTER SYSTEM SET UNDO_TABLESPACE = undotbs_02;
Assuming undotbs_01 is the current undo tablespace, after this command successfully executes, the instance uses undotbs_02 in place of undotbs_01 as its undo tablespace.
If any of the following conditions exist for the tablespace being switched to, an error is reported and no switching occurs:
The database is online while the switch operation is performed, and user transactions can be executed while this command is being executed. When the switch operation completes successfully, all transactions started after the switch operation began are assigned to transaction tables in the new undo tablespace.
The switch operation does not wait for transactions in the old undo tablespace to commit. If there are any pending transactions in the old undo tablespace, the old undo tablespace enters into a PENDING OFFLINE mode (status). In this mode, existing transactions can continue to execute, but undo records for new user transactions cannot be stored in this undo tablespace.
An undo tablespace can exist in this PENDING OFFLINE mode, even after the switch operation completes successfully. A PENDING OFFLINE undo tablespace cannot be used by another instance, nor can it be dropped. Eventually, after all active transactions have committed, the undo tablespace automatically goes from the PENDING OFFLINE mode to the OFFLINE mode. From then on, the undo tablespace is available for other instances (in an Oracle Real Application Cluster environment).
If the parameter value for UNDO TABLESPACE is set to '' (two single quotes), then the current undo tablespace is switched out and the next available undo tablespace is switched in. Use this statement with care, because if there is no undo tablespace available, the SYSTEM rollback segment is used. This causes ORA-01552 errors to be issued for any attempts to write non-SYSTEM related undo to the SYSTEM rollback segment.
The following example unassigns the current undo tablespace:ALTER SYSTEM SET UNDO_TABLESPACE = '';
Establishing User Quotas for Undo Space
The Oracle Database Resource Manager can be used to establish user quotas for undo space. The Database Resource Manager directive UNDO_POOL allows DBAs to limit the amount of undo space consumed by a group of users (resource consumer group).
You can specify an undo pool for each consumer group. An undo pool controls the amount of total undo that can be generated by a consumer group. When the total undo generated by a consumer group exceeds its undo limit, the current UPDATE transaction generating the redo is terminated. No other members of the consumer group can perform further updates until undo space is freed from the pool.
When no UNDO_POOL directive is explicitly defined, users are allowed unlimited undo space.
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