Introduction to Automatic Undo Management

This section introduces the concepts of Automatic Undo Management and discusses the following topics:

Overview of Automatic Undo Management
About the Undo Retention Period

Overview of Automatic Undo Management

Oracle provides a fully automated mechanism, referred to as automatic undo management, for managing undo information and space. With automatic undo management, the database manages undo segments in an undo tablespace. Beginning with Release 11g, automatic undo management is the default mode for a newly installed database. An auto-extending undo tablespace named UNDOTBS1 is automatically created when you create the database with Database Configuration Assistant (DBCA).

An undo tablespace can also be created explicitly. The methods of creating an undo tablespace are explained in "Creating an Undo Tablespace".

When the instance starts, the database automatically selects the first available undo tablespace. If no undo tablespace is available, the instance starts without an undo tablespace and stores undo records in the SYSTEM tablespace. This is not recommended, and an alert message is written to the alert log file to warn that the system is running without an undo tablespace.

If the database contains multiple undo tablespaces, you can optionally specify at startup that you want to use a specific undo tablespace. This is done by setting the UNDO_TABLESPACE initialization parameter, as shown in this example:

UNDO_TABLESPACE = undotbs_01

If the tablespace specified in the initialization parameter does not exist, the STARTUP command fails. The UNDO_TABLESPACE parameter can be used to assign a specific undo tablespace to an instance in an Oracle Real Application Clusters environment.

The database can also run in manual undo management mode. In this mode, undo space is managed through rollback segments, and no undo tablespace is used.

Note:

Space management for rollback segments is complex. Oracle strongly recommends leaving the database in automatic undo management mode.

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

Initialization Parameter	Description
`UNDO_MANAGEMENT`	If `AUTO` or null, enables automatic undo management. If `MANUAL`, sets manual undo management mode. The default is `AUTO`.
`UNDO_TABLESPACE`	Optional, and valid only in automatic undo management mode. Specifies the name of an undo tablespace. Use only when the database has multiple undo tablespaces and you want to direct the database instance to use a particular undo tablespace.

When automatic undo management is enabled, if the initialization parameter file contains parameters relating to manual undo management, they are ignored.

Note:

Earlier releases of Oracle Database default to manual undo management mode. To change to automatic undo management, you must first create an undo tablespace and then change the UNDO_MANAGEMENT initialization parameter to AUTO. If your Oracle Database is release 9i or later and you want to change to automatic undo management, see Oracle Database Upgrade Guide for instructions.

A null UNDO_MANAGEMENT initialization parameter defaults to automatic undo management mode in Release 11g and later, but defaults to manual undo management mode in earlier releases. You must therefore use caution when upgrading a previous release to Release 11g. Oracle Database Upgrade Guide describes the correct method of migrating to automatic undo management mode, including information on how to size the undo tablespace.

About the Undo Retention Period

After a transaction is committed, undo data is no longer needed for rollback or transaction recovery purposes. However, for consistent read purposes, long-running queries may require this old undo information for producing older images of data blocks. Furthermore, the success of several Oracle Flashback features can also depend upon the availability of older undo information. For these reasons, it is desirable to retain the old undo information for as long as possible.

When automatic undo management is enabled, there is always a current undo retention period, which is the minimum amount of time that Oracle Database attempts to retain old undo information before overwriting it. Old (committed) undo information that is older than the current undo retention period is said to be expired and its space is available to be overwritten by new transactions. Old undo information with an age that is less than the current undo retention period is said to be unexpired and is retained for consistent read and Oracle Flashback operations.

Oracle Database automatically tunes the undo retention period based on undo tablespace size and system activity. You can optionally specify a minimum undo retention period (in seconds) by setting the UNDO_RETENTION initialization parameter. The exact impact this parameter on undo retention is as follows:

The UNDO_RETENTION parameter is ignored for a fixed size undo tablespace. The database always tunes the undo retention period for the best possible retention, based on system activity and undo tablespace size. See "Automatic Tuning of Undo Retention" for more information.
For an undo tablespace with the AUTOEXTEND option enabled, the database attempts to honor the minimum retention period specified by UNDO_RETENTION. When space is low, instead of overwriting unexpired undo information, the tablespace auto-extends. If the MAXSIZE clause is specified for an auto-extending undo tablespace, when the maximum size is reached, the database may begin to overwrite unexpired undo information. The UNDOTBS1 tablespace that is automatically created by DBCA is auto-extending.

Automatic Tuning of Undo Retention

Oracle Database automatically tunes the undo retention period based on how the undo tablespace is configured.

If the undo tablespace is configured with the AUTOEXTEND option, the database dynamically tunes the undo retention period to be somewhat longer than the longest-running active query on the system. However, this retention period may be insufficient to accommodate Oracle Flashback operations. Oracle Flashback operations resulting in snapshot too old errors are the indicator that you must intervene to ensure that sufficient undo data is retained to support these operations. To better accommodate Oracle Flashback features, you can either set the UNDO_RETENTION parameter to a value equal to the longest expected Oracle Flashback operation, or you can change the undo tablespace to fixed size.
If the undo tablespace is fixed size, the database dynamically tunes the undo retention period for the best possible retention for that tablespace size and the current system load. This best possible retention time is typically significantly greater than the duration of the longest-running active query.

If you decide to change the undo tablespace to fixed-size, you must choose a tablespace size that is sufficiently large. If you choose an undo tablespace size that is too small, the following two errors could occur:
- DML could fail because there is not enough space to accommodate undo for new transactions.
- Long-running queries could fail with a snapshot too old error, which means that there was insufficient undo data for read consistency.
See "Sizing a Fixed-Size Undo Tablespace" for more information.

Note:

Automatic tuning of undo retention is not supported for LOBs. This is because undo information for LOBs is stored in the segment itself and not in the undo tablespace. For LOBs, the database attempts to honor the minimum undo retention period specified by UNDO_RETENTION. However, if space becomes low, unexpired LOB undo information may be overwritten.

Retention Guarantee

To guarantee the success of long-running queries or Oracle Flashback operations, you can enable retention guarantee. If retention guarantee is enabled, the specified minimum undo retention is guaranteed; the database never overwrites unexpired undo data even if it means that transactions fail due to lack of space in the undo tablespace. If retention guarantee is not enabled, the database can overwrite unexpired undo when space is low, thus lowering the undo retention for the system. This option is disabled by default.

WARNING:

Enabling retention guarantee can cause multiple DML operations to fail. Use with caution.

You enable retention guarantee by specifying the RETENTION GUARANTEE clause for the undo tablespace when you create it with either the CREATE DATABASE or CREATE UNDO TABLESPACE statement. Or, you can later specify this clause in an ALTER TABLESPACE statement. You disable retention guarantee with the RETENTION NOGUARANTEE clause.

You can use the DBA_TABLESPACES view to determine the retention guarantee setting for the undo tablespace. A column named RETENTION contains a value of GUARANTEE, NOGUARANTEE, or NOT APPLY, where NOT APPLY is used for tablespaces other than the undo tablespace.

Undo Retention Tuning and Alert Thresholds

For a fixed-size undo tablespace, the database calculates the best possible retention based on database statistics and on the size of the undo tablespace. For optimal undo management, rather than tuning based on 100% of the tablespace size, the database tunes the undo retention period based on 85% of the tablespace size, or on the warning alert threshold percentage for space used, whichever is lower. (The warning alert threshold defaults to 85%, but can be changed.) Therefore, if you set the warning alert threshold of the undo tablespace below 85%, this may reduce the tuned size of the undo retention period. For more information on tablespace alert thresholds, see "Managing Tablespace Alerts".

Tracking the Tuned Undo Retention Period

You can determine the current retention period by querying the TUNED_UNDORETENTION column of the V$UNDOSTAT view. This view contains one row for each 10-minute statistics collection interval over the last 4 days. (Beyond 4 days, the data is available in the DBA_HIST_UNDOSTAT view.) TUNED_UNDORETENTION is given in seconds.

select to_char(begin_time, 'DD-MON-RR HH24:MI') begin_time,
to_char(end_time, 'DD-MON-RR HH24:MI') end_time, tuned_undoretention
from v$undostat order by end_time;

BEGIN_TIME      END_TIME        TUNED_UNDORETENTION
--------------- --------------- -------------------
04-FEB-05 00:01 04-FEB-05 00:11               12100
      ...                                          
07-FEB-05 23:21 07-FEB-05 23:31               86700
07-FEB-05 23:31 07-FEB-05 23:41               86700
07-FEB-05 23:41 07-FEB-05 23:51               86700
07-FEB-05 23:51 07-FEB-05 23:52               86700

576 rows selected.

See Oracle Database Reference for more information about V$UNDOSTAT.