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Guaranteed undo retention simply means that Oracle will retain undo data for the entire length of the undo retention period you specify, no matter what. That is, if you specify half an hour as the undo retention interval, Oracle will retain the undo data for the full 30 minutes, under all circumstances. If you run out of room for recording the undo information generated by new transactions, any new DML transactions will fail, since Oracle won t be able to store the undo information for those changes. Thus, there is a trade-off between guaranteeing undo information and the potential failure of some DML statements.

By default, Oracle doesn t guarantee undo retention; the default retention time is 900 seconds (15 minutes) if you decide to guarantee undo retention.

You can specify guaranteed undo retention for the undo tablespace when you create the database, or by specifying the RETENTION GUARANTEE clause while creating a new undo tablespace, as shown here: SQL> CREATE UNDO TABLESPACE undotbs01 2 DATAFILE 3 '/u01/orcl/oradata/undotbs01_01.dbf' 4 SIZE 10M AUTOEXTEND ON 5* RETENTION GUARANTEE; Tablespace created. SQL> You can also use the ALTER TABLESPACE command to guarantee undo retention in your database, as shown here: SQL> ALTER TABLESPACE undotbs01 RETENTION GUARANTEE; You can use the RETENTION NOGUARANTEE clause to turn off the guaranteed retention of undo information, as shown in the following example: SQL> ALTER TABLESPACE undotbs01 RETENTION NOGUARANTEE;

A high value for the UNDO_RETENTION parameter doesn t guarantee the retention of undo data for the duration specified by the UNDO_RETENTION parameter. You must use the RETENTION GUARANTEE clause to guarantee undo retention for a specified time.

Let s say you ve configured guaranteed undo retention in your database by using the RETENTION GUARANTEE clause. If your undo tablespace is too small to accommodate all the active transactions that are using it, the following will happen: Oracle will issue an automatic tablespace warning alert when the undo tablespace is 85 percent full (if you haven t disabled the automatic tablespace alert feature). Oracle will also issue an automatic tablespace critical alert when the undo tablespace is 97 percent full. All DML statements will be disallowed and will receive an out-of-space error. DDL statements will continue to be allowed.

Managing undo tablespaces is similar to managing any other tablespaces in your database. You add space to an undo tablespace by adding a datafile, and you decrease the size of an undo tablespace by reducing the size of the datafile(s) in it with the ALTER DATABASE DATAFILE . . . RESIZE command. You drop an undo tablespace with the normal DROP TABLESPACE command. (If the undo tablespace contains any outstanding transactions, you can t drop it.) The DROP TABLESPACE command, since it removes all contents of the undo tablespace, is similar to using the DROP TABLESPACE . . . WITH CONTENTS command. If you need to switch undo tablespaces for some reason, you can drop the old one after you create a new undo tablespace.

Occasionally, a long-running transaction can t find the undo data it needs, and consequently fails with the well-known Oracle snapshot-too-old error. Here s an example: SQL> 2 3 4 BEGIN purge_data_pkg.main_driver(1502,2005,'N','B','N','N'); END; /

begin * ERROR at line 1: ORA-01555: snapshot too old: rollback segment number 9 with name "_SYSSMU9$" too small ORA-06512: at "APPOWNER.PURGE_DATA_PKG", line 2040 ORA-06512: at "APPOWNER.PURGE_DATA_PKG", line 4318 ORA-06512: at line 2 SQL> Even when you use Automatic Undo Management, as the previous example shows, you can get this error, since the UNDO_RETENTION parameter is set too low. This happens even when there is plenty of free space in the undo tablespace. Your best bet is to raise the value of the UNDO_RETENTION parameter so the necessary undo data isn t overwritten before your long transaction finishes. The only certain way to avoid the snapshot-too-old error is to enable guaranteed undo retention in your database.