Re: Rollback Segement vs Redo log

Hi ,
Here is the brief description of rollback segments and redologs

Rollback : When the user or the server (at times of recovery) want to undo some changes that are made to the database, it undo the changes based on the information from rollback segments. For example when you delete a row , the server write the details into rollback segment (rowid and the data). But if you issue a rollback instead of commit , oracle server will write back the data from the rollback segment . I agree the data is available in the redo log also. But the changes in the rollback segment is available only till the transaction is complete. Once you do a commit or rollback it will not be there in rollback segment.
( redo log only will have the rowid of the deleted row which is true only for the moment of the time the row was deleted. If you want to rollback , you need the data not just the rowid.) .

Redolog : As the name implies it is used for the purposes of recovery.It contains all the transactions that are taking place in the database in the sequence in which it takes place.In case of crashes , If you have the backup of old database you can replay all the transactions from the archived logs( they are basically copy of redo logs) .Oracle also uses the redo logs for automatic recovery of block corruptions and from minor crashes/sudden power down etc.

Redo logs also is used when you take a hot backup to store the changes made in the datafile which is being backed up.

The purpose of redo log to perform the successful the job again for recovery purposes at any time.
The purpose of rollback is to facililate undoing the changes within the transaction only.

Regards
Anbu

John C. <john_at_rte-asset.com> wrote in article <33D6CAF9.60F_at_rte-asset.com>...
> Hi, there
>
> Can some one explain the functional differences between rollback segment
 and
> redo log in a brief description?
>
> I am still confused by theses two things and got more confused when I
 read
> concept manual from Oracle.
>
> Redo and rollback? sounds like the same. thanks in advance for your
 feedback
>
>
> --
> John Chan
> Manager, Data Services
> RTE Asset Management (aka. Rightime Econometrics, Inc.)
> 1095 Rydal Rd.
> Rydal, PA 19046-1711
> 215-572-7288 (Voice)
> 215-572-6254 (Fax)
> Internet address: john_at_rte-asset.com
>
Received on Thu Jul 24 1997 - 00:00:00 CDT