What is an undo segment?

What is an undo segment?

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An undo segment holds all undo data of a database when the database is running in automatic undo management mode. An undo segment is internally similar to a rollback segment that is used to hold undo data of a database when the database is running in manual undo management mode.

When a transaction modifies data in a database, the before image of the modified data is stored in an undo segment. If the transaction is rolled back, Oracle restores the original values by writing the values stored in the undo segment back to the modified rows.

During an instance recovery, the values stored in an undo segment are used to roll back the uncommitted transactions. The values stored in an undo segment also provide the transaction-level or statement-level read consistency, i.e., database users are able to see a consistent image of data even if the data is being modified by another user at the same time.

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