TAILIEUCHUNG - Managing Undo Data

Managing Undo Data Automatic Undo Management The Oracle server automatically manages the creation, allocation, and tuning of undo segments. Manual Undo Management You manually manage the creation, allocation, and tuning of undo segments. It was the only method available prior to Oracle9i. Information about manual undo management can be found in “Appendix B: Manually Managing Undo Data.” | Managing Undo Data Objectives After completing this lesson, you should be able to do the following: Describe the purpose of undo data Implement Automatic Undo Management Create and configure undo segments Obtain undo segment information Managing Undo Data There are two methods for managing undo data: Automatic Undo Management Manual Undo Management The term undo was known as rollback in previous versions. Managing Undo Data Automatic Undo Management The Oracle server automatically manages the creation, allocation, and tuning of undo segments. Manual Undo Management You manually manage the creation, allocation, and tuning of undo segments. It was the only method available prior to Oracle9i. Information about manual undo management can be found in “Appendix B: Manually Managing Undo Data.” Undo Segment Update transaction Old image New image Undo segment Table Undo Segment An undo segment is used to save the old value (undo data) when a process changes data in a database. It stores the location of the data and the data as it existed before being modified. The header of an undo segment contains a transaction table where information about the current transactions using the undo segment is stored. A serial transaction uses only one undo segment to store all of its undo data. Many concurrent transactions can write to one undo segment. Undo Segments: Purpose Transaction rollback Transaction recovery Undo segment Read consistency Undo Segments: Purpose Transaction Rollback When a transaction modifies a row in a table, the old image of the modified columns (undo data) is saved in the undo segment. If the transaction is rolled back, the Oracle server restores the original values by writing the values in the undo segment back to the row. Transaction Recovery If the instance fails while transactions are in progress, the Oracle server needs to undo any uncommitted changes when the database is opened again. This rollback is part of transaction recovery. Recovery is possible only

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