tailieunhanh - Tuning Undo Segments

After completing this lesson, you should be able to do the following: Describe the concept of automatic undo management Create and maintain the automatic managed undo tablespace Set the retention period Use dynamic performance views to check rollback segment performance Reconfigure and monitor rollback segments Define the number and sizes of rollback segments Allocate rollback segments to transactions | Tuning Undo Segments Objectives After completing this lesson, you should be able to do the following: Describe the concept of automatic undo management Create and maintain the automatic managed undo tablespace Set the retention period Use dynamic performance views to check rollback segment performance Reconfigure and monitor rollback segments Define the number and sizes of rollback segments Allocate rollback segments to transactions Objectives This lesson helps you understand the automatic undo management feature. You will also learn to configure automatic undo management in an Oracle database. Good rollback segment configuration is crucial to a well-tuned Oracle database. This lesson helps you to recognize and solve problems arising from inappropriate numbers or sizes of rollback segments. Undo Segments: Usage Transaction rollback Transaction recovery Undo (Rollback) segment Read consistency Control files Data files Redo log files Undo Segments: Usage Transaction Rollback When a . | Tuning Undo Segments Objectives After completing this lesson, you should be able to do the following: Describe the concept of automatic undo management Create and maintain the automatic managed undo tablespace Set the retention period Use dynamic performance views to check rollback segment performance Reconfigure and monitor rollback segments Define the number and sizes of rollback segments Allocate rollback segments to transactions Objectives This lesson helps you understand the automatic undo management feature. You will also learn to configure automatic undo management in an Oracle database. Good rollback segment configuration is crucial to a well-tuned Oracle database. This lesson helps you to recognize and solve problems arising from inappropriate numbers or sizes of rollback segments. Undo Segments: Usage Transaction rollback Transaction recovery Undo (Rollback) segment Read consistency Control files Data files Redo log files Undo Segments: Usage Transaction Rollback When a transaction makes changes to a row in a table, the old image is saved in an undo segment, also called a rollback segment. If the transaction is rolled back, the value in the undo segment is written back to the row, restoring the original value. Transaction Recovery If the instance fails when transactions are in progress, then the Oracle server rolls back the uncommitted changes when the instance is restarted. Read Consistency Read consistency consists of the following conditions: When transactions are in progress, other sessions in the database should not see any uncommitted changes. A query statement should not see any changes that are made, committed or uncommitted, after the statement commenced execution. DML statements do not see any uncommitted changes. However, the changed rows are locked by the transaction making the changes and the lock is released only when the transaction ends. The old values in the undo segments, also referred to as undo information, are used to provide the .