What is UNDO DATA?

What is UNDO DATA?

UNDO DATA is the copy of pre-modified data that is captured for every transactions that changes data

Why UNDO required?

* rollback transactions
* support read consistency
* support flashback operations
* recover from failed transactions

How long does UNDO data stay ?

It is retained until one of the following happens:

* users commits the transactions
* users rollback the transactions
* user execute the DDL statements (CREATE, DROP, ALTER, RENAME)
* user session is terminated abnormally (transactions rollback)
* user session terminates normally with an exit (transactions commits)

Where does UNDO data stay ?

* Stored in UNDO segments/UNDO tablespace
* Only 1 active tablespace for an instance
* They are owned by the user SYS