Showing posts with label Definitions. Show all posts
Showing posts with label Definitions. Show all posts

Tuesday, January 29, 2019

The difference between REDO and UNDO


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