Confusion about recovery

Question

Hi ,
I am new to the DBA field and I have a confusion about recovery.
The confusion is if a database is in noarchivelog mode can a database be recovered from commited changes that were there in the redo log files ?
If I provide the path name of the redo log files while using recover database using cancel will it work at all given that the database is in noarchivelog mode ?
Please help to clear my doubts..

Answer 1

Oracle can use the Online Redo Logs for Recovery. Normally this happens in the case of Instance Recovery (e.g. from a server crash or shutdown abort) -- where the datafiles are not restored from a prior backup.
If you restore datafiles from a prior backup, you are doing a media recovery. In NOARCHIVELOG mode, you could not have run a backup with the database OPEN, so the backup would have been run with the database SHUTDOWN or MOUNTed. At the subsequent startup, transactions would be in the online redo logs only until LGWR does a "wrap around" and overwrites the first redo log used after the startup. It is only within this window that transactions are in the redo logs.+
Remember that LGWR uses a "round-robin" algorithm to cycle through the online redo logs. So, if the Online Redo Log that was CURRENT at the time of the backup has been overwritten, you cannot use the Online Redo Logs for a RECOVERy+._
You must also ensure that there are no NOLOGGING operations !!
One thing that you might trip up on is the behaviour of CTAS. A "CREATE TABLE AS SELECT" is, by default LOGGING in an ARCHIVELOG database. However, it is automatically a Direct Path operation in a NOARCHIVELOG database ! So the blocks for such a table would be "corrupt" if you attempt a recovery from the Online Redo Log as the row inserts are not captured.
Hemant K Chitale
Edited by: Hemant K Chitale on Oct 10, 2011 11:43 AM
Edited by: Hemant K Chitale on Oct 10, 2011 11:44 AM
Edited by: Hemant K Chitale on Oct 10, 2011 4:33 PM
Edited by: Hemant K Chitale on Oct 10, 2011 4:34 PM