What is undo space?

What is undo space?

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The undo space consists of records that are maintained to rollback any change to the database. Primarily, the information in an undo space consists of records of changes made by the transactions before they were being committed. These records are known as the undo records. The data contained in these records are used to undo the changes made by a transaction, recover a database from failure, and provide read consistency.

The undo records can either be stored in the rollback segments or the undo tablespace. Using rollback segments for storing the undo records comes under the manual method of storage. This method is quite complex. The automatic method of managing undo space uses an undo tablespace for storing the undo records. The mode of undo space management can be specified by the UNDO_MANAGEMENT parameter. When this parameter is set to AUTO, the undo space is managed automatically. The default undo tablespace is created when the instance starts. However, if Oracle is unable to find the default undo tablespace, it uses the SYSTEM rollback segment, though this is not recommended under normal circumstances.

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