Some other important facts

In this section we will talk about some other important facts about redo that are important for you to know.

Redo and undo for DML

When you issue an insert, update, or delete, Oracle actually makes the change to the data blocks that contain the affected data even though you have not issued a commit. To ensure database integrity, Oracle must write information necessary to reverse the change (undo) into the redo log file to handle any transaction failure or even a rollback. Recovery from media failure is ensured by writing the information necessary to replay all database changes (redo) to the database into the redo log file. So, undo and redo information needs to be written into the transaction log of the RDBMS as a logical consequence to protect the integrity of the data.

While the RDBMS logically would only need to write undo and redo into the transaction log, the undo portion must also be kept online (on disk and accessible to the RDBMS engine) to enable rollback...