Oracle® Database Administrator's Guide 11g Release 1 (11.1) Part Number B28310-04 |
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This section describes the various steps involved in undo tablespace management and contains the following sections:
Although Database Configuration Assistant (DBCA) automatically creates an undo tablespace for new installations of Oracle Database Release 11g, there may be occasions when you want to manually create 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".
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.
CREATE DATABASE rbdb1 CONTROLFILE REUSE . . . UNDO TABLESPACE undotbs_01 DATAFILE '/u01/oracle/rbdb1/undo0101.dbf';
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. This is discussed in "Supporting Bigfile Tablespaces During Database Creation".
See Also:
Oracle Database SQL Language Reference for the syntax for using theCREATE DATABASE
statement to create an undo tablespaceThe 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 with the AUTOEXTEND
option:
CREATE UNDO TABLESPACE undotbs_02 DATAFILE '/u01/oracle/rbdb1/undo0201.dbf' SIZE 2M REUSE AUTOEXTEND ON;
You can create more than one undo tablespace, but only one of them can be active at any one time.
See Also:
Oracle Database SQL Language Reference for the syntax for using theCREATE UNDO TABLESPACE
statement to create an undo tablespaceUndo 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.
See Also:
Oracle Database SQL Language Reference for ALTER TABLESPACE
syntax
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.
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 there may be no undo tablespace available.
The following example unassigns the current undo tablespace:
ALTER SYSTEM SET UNDO_TABLESPACE = '';
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 undo 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.
Oracle Database also provides proactive help in managing tablespace disk space use by alerting you when tablespaces run low on available space. Please refer to "Managing Tablespace Alerts" for information on how to set alert thresholds for the undo tablespace.
In addition to the proactive undo space alerts, Oracle Database also provides alerts if your system has long-running queries that cause SNAPSHOT
TOO
OLD
errors. To prevent excessive alerts, the long query alert is issued at most once every 24 hours. When the alert is generated, you can check the Undo Advisor Page of Enterprise Manager to get more information about the undo tablespace.