9-Managing Undo Data (2)

9-Managing Undo Data (2) - anaging Undo Data Tathagata...

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Unformatted text preview: anaging Undo Data Tathagata Bhattacharjee Managing Undo Data Presenter Tathagata Bhattacharjee anaging Undo Data Tathagata Bhattacharjee Objectives After completing this lesson you should be able to do the following Describe the purpose of undo data Implement Automatic Undo Management Obtain undo segment information from the data dictionary anaging Undo Data Tathagata Bhattacharjee Managing Undo Data Overview Two methods for managing undo data exists Automatic Undo Management The Oracle Server automatically manages the creation, allocation and tuning of undo segments Manual Undo Management You may manually manage the creation, allocation and tuning of undo segments. The term undo replaces what was known in previous versions of Oracle as rollback anaging Undo Data Tathagata Bhattacharjee Undo Segment Old Image Undo Segment Update Transaction New image anaging Undo Data Tathagata Bhattacharjee An Undo Segment is used to save the old value (undo data) when processes 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 anaging Undo Data Tathagata Bhattacharjee Purpose of Undo Segments 1. 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. anaging Undo Data Tathagata Bhattacharjee 2. 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....
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9-Managing Undo Data (2) - anaging Undo Data Tathagata...

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