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This article mainly explains "what is the difference between UNDO and REDO". The content in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "what is the difference between UNDO and REDO".

UNDO in order to solve the problem of consistent read (consistent get) and rollback

REDO is for the consideration of database security, the operation of the DML of the database will be redo file.

The setting of the UNDO parameter in the data can be viewed using show parameter undo

Undo_management indicates that the rollback segment is managed automatically.

Undo_retention 900indicates that the front image retains 900s after COMMIT and can be overwritten. The main purpose is to complete the query time and initiate an earlier statement to obtain the data in the previous image file, so as to avoid) the ORA-01555 rollback segment is too old.

Undo_tablespace represents the tablespace in which the rollback segment resides.

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