TAILIEUCHUNG - Syngress Managing Undo Data

After completing this lesson, you should be able to do the following: • Explain DML and undo data generation • Monitor and administer undo data • Describe the difference between undo data and redo data • Configure undo retention • Guarantee undo retention • Use the Undo Advisor | Managing Undo Data Objectives After completing this lesson, you should be able to do the following: Explain DML and undo data generation Monitor and administer undo data Describe the difference between undo data and redo data Configure undo retention Guarantee undo retention Use the Undo Advisor Data Manipulation Data manipulation language (DML) consists of the following SQL statements: INSERT UPDATE DELETE MERGE DML always executes as part of a transaction, which can be: Rolled back, using the ROLLBACK command Committed, using the COMMIT command Data Manipulation Data is manipulated, or modified, by the DML class of SQL statements: INSERT, UPDATE, DELETE, and MERGE. These statements execute as part of a transaction, which starts with the first successful DML statement and ends with either a COMMIT or ROLLBACK command. A transaction is either entirely committed or entirely rolled back. Rollback may also occur if there is a process or system failure. Note: The MERGE command performs a combination of inserts and updates to merge data from one table into another. It is covered in the lesson titled “Managing Data and Concurrency.” Undo Data Undo data is: A copy of original, premodified data Captured for every transaction that changes data Retained at least until the transaction is ended Used to support: Rollback operations Read-consistent and flashback queries Recovery from failed transactions User Undo Data The Oracle database saves the old value (undo data) when a process changes data in a database. It stores the data as it existed before being modified. Capturing undo data enables you to roll back your uncommitted data. Undo also supports read-consistent and flashback queries. Read-consistent queries provide results that are consistent with the data as of the time a query started. For a read-consistent query to succeed, the original information must still exist as undo information. As long as the undo information is retained, the Oracle database can reconstruct data

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