Undo vs Redo

 Here is a simple difference between the two:

Undo	Redo
Makes a change go away	Reproduces a change
Used for rollback and read consistency	Used for rolling forward the changes
Protects the database from inconsistent reads	Protects from data loss
Data stored in Undo segments in Undo tablespace	Logs stored in SGA, in memory

 

Undo

1.       Undo is a record of a transaction before it is committed.

2.       When issuing a ROLLBACK statement, undo records help to undo changes made to the database by the uncommitted transaction.

3.       We use Undo for rollback and read consistency

Redo 

1.       On the other hand, redo is an entry in redo log files that holds a group of change vectors.

2.       These logs contain files with a history of all changes made to the database.

3.       Redo for rolling forward database changes