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The control file maintains an SCN value for each data file, called the STOP SCN, which is usually set to infinity, and every time the instance is stopped normally (with the SHUTDOWN NORMAL or SHUTDOWN IMMEDIATE command), Oracle copies the START SCN number in the data file headers to the STOP SCN numbers for the data files in the control file When you restart the database after a graceful shutdown, there is no need for any kind of recovery because the SCNs in the data files and the control files match On the other hand, abrupt instance termination does not leave time for this matching of SCNs, and Oracle recognizes that instance recovery is required because of the varying SCN numbers in the data files on the one hand and the control file on the other As you ll learn in 16, they play a critical role during database recovery.

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Oracle determines how far back you should apply the archived redo logs during a recovery based on the SCN..

When you make a change to a table, you should be able to undo or roll back the change if necessary The information needed to undo or roll back changes in transactions, which mostly consists of the pre-change table row information, is called undo data (the change vectors), and it is stored in undo records When you issue a ROLLBACK command, Oracle uses these undo records to replace the changed data with the original versions As 6 explains in detail, the undo records are vital during database recovery when all unfinished or uncommitted transactions must be discarded to make the database consistent In earlier versions of the Oracle database (up to Oracle8i), it was the DBA s job to manage what are known as rollback segments by explicitly allocating a regular, permanent tablespace for them.

In fact, the management of the rollback segments used to be a vexing and time-consuming part of the job for many DBAs who managed large databases, especially if they had frequent, long-running transactions Oracle wrote (and still writes) to the rollback segments in a circular fashion, and it wasn t uncommon to find that information needed by a transaction for read consistency had been overwritten by a newer transaction Many DBAs used to find the sizing of the rollback segments a tricky issue: if you had several small rollback segments, a large transaction might fail, and if you had a small number of very large rollback segments, your transactions might encounter contention for the rollback segment transaction tables The manual mode of undo management is deprecated in Oracle Database 10g.

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Oracle strongly recommends the use of the Automatic Undo Management (AUM) feature, where the Oracle server itself will maintain and manage the undo (rollback) segments All you need to do is provide a dedicated undo tablespace and set the initialization parameter UNDO_MANAGEMENT to auto The default setting of the UNDO_MANAGEMENT parameter is still manual in Oracle Database 10g Oracle will create the necessary number of undo segments, which are structurally similar to the traditional rollback segments, and it ll size and extend them as necessary It s not uncommon for new undo segments to be created and old ones to be deactivated based on the amount of transactions going on in the database 6 provides further information about the AUM feature.

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Because Oracle will do the sizing of the individual undo segments for you, the two decisions you have to make are the size of the undo tablespace and the setting for the UNDO_RETENTION initialization parameter (which determines how long Oracle will try to retain undo records in the undo tablespace) Remember that your undo tablespace should not only be able to accommodate all the long-running transactions, but it also has to be big enough to accommodate any flashback features you may implement in your database Oracle s flashback features let you undo changes to data at various levels Several flashback features, such as Flashback Query, Flashback Versions Query, and Flashback Table, utilize undo data I discuss the undo-related Flashback features in 6.