10 Managing the Undo Tablespace

Overview of Automatic Undo Management

In past releases, when you used the rollback segment method of managing undo space, you were said to be operating in the manual undo management mode. Now, you use the undo tablespace method, and you are said to be operating in the automatic undo management mode. You determine the mode at instance startup using the UNDO_MANAGEMENT initialization parameter. The default value for this parameter is MANUAL. You set it to AUTO to enable automatic undo management.

The following initialization parameter setting causes the STARTUP command to start an instance in automatic undo management mode:

UNDO_MANAGEMENT = AUTO

An undo tablespace must be available, into which the database will store undo records. The default undo tablespace is created at database creation, or an undo tablespace can be created explicitly. The methods of creating an undo tablespace are explained in "Creating an Undo Tablespace"

When the instance starts up, the database automatically selects for use the first available undo tablespace. If there is no undo tablespace available, the instance starts, but uses the SYSTEM rollback segment for undo. This is not recommended in normal circumstances, and an alert message is written to the alert file to warn that the system is running without an undo tablespace. ORA-01552 errors are issued for any attempts to write non-SYSTEM related undo to the SYSTEM rollback segment.

If the database contains multiple undo tablespaces, you can optionally specify at startup that you want an Oracle Database instance to use a specific undo tablespace. This is done by setting the UNDO_TABLESPACE initialization parameter. For example:

UNDO_TABLESPACE = undotbs_01

In this case, if you have not already created the undo tablespace (in this example, undotbs_01), the STARTUP command will fail. The UNDO_TABLESPACE parameter can be used to assign a specific undo tablespace to an instance in an Oracle Real Application Clusters environment.

The following is a summary of the initialization parameters for automatic undo management mode:

If the initialization parameter file contains parameters relating to manual undo management, they are ignored.

Undo Retention

Committed undo information normally is lost when its undo space is overwritten by a newer transaction. However, for consistent read purposes, long-running queries sometimes require old undo information for undoing changes and producing older images of data blocks. The success of several Flashback features can also depend upon older undo information.

UNDO_RETENTION = 1800

ALTER SYSTEM SET UNDO_RETENTION = 2400

Retention Guarantee

Oracle Database 10g lets you guarantee undo retention. When you enable this option, the database never overwrites unexpired undo data--that is, undo data whose age is less than the undo retention period. This option is disabled by default, which means that the database can overwrite the unexpired undo data in order to avoid failure of DML operations if there is not enough free space left in the undo tablespace.

By enabling the guarantee option, you instruct the database not to overwrite unexpired undo data even if it means risking failure of currently active DML operations. Therefore, use caution when using this feature. A typical use of the guarantee option is when you want to ensure deterministic and predictable behavior of Flashback Query by guaranteeing the availability of the required undo data.

You enable the guarantee option by specifying the RETENTION GUARANTEE clause for the undo tablespace when it is created by either the CREATE DATABASE or CREATE UNDO TABLESPACE statement. Or, you can later specify this clause in an ALTER TABLESPACE statement. You do not guarantee that unexpired undo is preserved if you specify the RETENTION NOGUARANTEE clause.

You can use the DBA_TABLESPACES view to determine the RETENTION setting for the undo tablespace. A column named RETENTION will contain a value on GUARANTEE, NOGUARANTEE, or NOT APPLY (used for tablespaces other than the undo tablespace).

Using Auto-Extensible Tablespaces

Oracle Database supports automatic extension of the undo tablespace to facilitate capacity planning of the undo tablespace in the production environment. When the system is first running in the production environment, you may be unsure of the space requirements of the undo tablespace. In this case, you can enable automatic extension for datafiles of the undo tablespace so that they automatically increase in size when more space is needed. By combining automatic extension of the undo tablespace with automatically tuned undo retention, you can ensure that long-running queries will succeed by guaranteeing the undo required for such queries.

After the system has stabilized and you are more familiar with undo space requirements, Oracle recommends that you set the maximum size of the tablespace to be slightly (10%) more than the current size of the undo tablespace.

Sizing Fixed-Size Undo Tablespaces

If you have decided on a fixed-size undo tablespace, the Undo Advisor can help you estimate needed capacity, and you can then calculate the amount of retention your system will need. You can access the Undo Advisor through Enterprise Manager or through the DBMS_ADVISOR PL/SQL package. Enterprise Manager is the preferred method of accessing the advisor. For more information on using the Undo Advisor through EM, please refer to Oracle Database 2 Day DBA.

The Undo Advisor relies for its analysis on data collected in the Automatic Workload Repository (AWR). An adjustment to the collection interval and retention period for AWR statistics can affect the precision and the type of recommendations the advisor produces. Please refer to "Automatic Workload Repository" for additional information.

DECLARE
   tid    NUMBER;
   tname  VARCHAR2(30);
   oid    NUMBER;
   BEGIN
   DBMS_ADVISOR.CREATE_TASK('Undo Advisor', tid, tname, 'Undo Advisor Task');
   DBMS_ADVISOR.CREATE_OBJECT(tname, 'UNDO_TBS', null, null, null, 'null', oid);
   DBMS_ADVISOR.SET_TASK_PARAMETER(tname, 'TARGET_OBJECTS', oid);
   DBMS_ADVISOR.SET_TASK_PARAMETER(tname, 'START_SNAPSHOT', 1);
   DBMS_ADVISOR.SET_TASK_PARAMETER(tname, 'END_SNAPSHOT', 2);
   DBMS_ADVISOR.execute_task(tname);
   end;
/

UndoSpace = UR * UPS + overhead

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. Single-file, or bigfile, tablespaces are discussed in "Bigfile Tablespaces".

CREATE DATABASE rbdb1
     CONTROLFILE REUSE
     .
     .
     .
     UNDO TABLESPACE undotbs_01 DATAFILE '/u01/oracle/rbdb1/undo0101.dbf';

CREATE UNDO TABLESPACE undotbs_02
     DATAFILE '/u01/oracle/rbdb1/undo0201.dbf' SIZE 2M REUSE AUTOEXTEND ON;

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:

• Adding a datafile

• Renaming a datafile

• Bringing a datafile online or taking it offline

• Beginning or ending an open backup on a datafile

• Enabling and disabling undo retention guarantee

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:

ALTER TABLESPACE undotbs_01
     ADD DATAFILE '/u01/oracle/rbdb1/undo0102.dbf' AUTOEXTEND ON NEXT 1M 
         MAXSIZE UNLIMITED;

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 tablespace does not exist

• The tablespace is not an undo tablespace

• The tablespace is already being used by another instance (in a RAC environment only)

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.

Flashback Query

Using Oracle Flashback Query feature, users or applications can execute queries as of a previous time in the database. Application developers can use Flashback Query to design an application that allows users to correct their mistakes with minimal DBA intervention. You, as the DBA, need only configure the undo tablespace with an appropriate size and undo retention period. No further action on your part should be required.

The Oracle-supplied DBMS_FLASHBACK package implements Flashback Query at the session level. At the object level, Flashback Query uses the AS OF clause of the SELECT statement to specify the previous point in time for which you wish to view data.

Flashback Versions Query

The Flashback Version Query feature enables users to query the history of a given row. A SELECT statement that specifies a VERSIONS clause returns individual versions of a row between two specified SCNs (system change numbers) or timestamps. The UNDO_RETENTION initialization parameter must be set to a value that is large enough to cover the period of time specified in the VERSIONS clause. Otherwise, not all rows can be retrieved.

The VERSIONS clause can also be used in subqueries of DML and DDL statements.

Flashback Transaction Query

Oracle Database provides a view, FLASHBACK_TRANSACTION_QUERY, that enables you to identify changes made by a particular transaction, or by all transactions issued within a specified period of time. One use for this view could be if a user finds, by using the Flashback Transaction Query feature, that a row value has been changed inappropriately. Querying the FLASHBACK_TRANSACTION_QUERY view can provide specific details of the transaction or transactions that changed the row value.

Flashback Table

The FLASHBACK TABLE statement lets users recover a table to a previous point in time. It provides a fast, online solution for recovering a table that has been accidentally modified or deleted by a user or application. The UNDO_RETENTION initialization parameter must be set to a value that is large enough to cover the period specified in the FLASHBACK TABLE statement.