| UNDO | REDO |
|---|---|
| Used during normal database operations | Used during instance and media recovery only |
| Contains changed column values | Contains "change vectors", a description of a change made to a single block |
| Stored inside the database, in a separate tablespace | Stored outside the database, in redo log files |
| Can be utilized for many purposes | Has one purpose only |
The redo log files has one and only one purpose in life: instance recovery.
The undo tablespace(s) has one primary purpose in life: to support read-consistency. However, unlike redo, UNDO data can also be utilized for many other purposes, like Oracles flashback technologies.
Oracle 12.2 Documentation about the redo logs can be found here
