Does UNDO data persist after a bounce ?

Question

RDBMS Version : 11.2, 10.2
Platform : Unix
Will Oracle retain the Undo data in UNDO tablespace after a bounce ? If yes, why does oracle want to retain UNDO data which was present before the shutdown ? Is there a way to verify that UNDO data is retained after the bounce ?

Answer 1

Hi
It is probably better to understand the difference between undo and a transaction.
1. Undo data (more accurately undo records) - means undo records stored in the undo blocks of an undo tablespace. Retention of those undo records are controlled by undo_retention parameter (and also by auto undo retention tuning feature). Even if you restart the database, those undo records are still kept in undo tablespace (meaning, undo tablespace is not cleaned out); These undo records are used during Consistent Read copy generation and transaction rollback as discussed in (2) below.
2. Transactions - Shutdown immediate kills the sessions and rolls back active transactions; shutdown transactional will wait for the transaction to complete, but kills the session after completion of current active transaction ; shutdown normal will wait for the session to complete (and so transactions to complete); shutdown abort will kill the session, but does not roll back active transactions.
Further read: http://docs.oracle.com/cd/E11882_01/server.112/e10713/startup.htm#BABDBBAJ
( I really hope, your HR doesn't read this thread. )
Cheers!
Riyaj Shamsudeen
OakTable Member/Oracle ACE Director
Blog: http://orainternals.wordpress.com
Fall 2012: [Advanced RAC seminar | http://www.orainternals.com/services/training/advanced-rac-training/]