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Oracle Database Transactions and Locking Revealed by Darl Kuhn, Thomas Kyte

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CHAPTER 6

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Redo and Undo

This chapter introduces you two of the most important pieces of data in an Oracle database: redo and undo. Redo is the information Oracle records in online (and archived) redo log files in order to “replay” your transaction in the event of a failure. Undo is the information Oracle records in the undo segments in order to reverse, or roll back, your transaction.

In this chapter, we will discuss topics such as how redo and undo (rollback) are generated, and how they fit into transactions. We’ll also discuss the performance implications of COMMIT and ROLLBACK statements. I will present the pseudo-code for these mechanisms ...

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