Re: Rollback Segement vs Redo log

From: Thomas Kyte <tkyte_at_us.oracle.com>
Date: 1997/07/24
Message-ID: <33dbb5d2.19138699@newshost>#1/1

rollback = undo, undo a transaction, restore the blocks to what they were.

redo = redo, do it again, make the blocks what they should be.

rollback is used for 2 purposes:

• to rollback a transaction and reset the affected blocks to what they were before the transaction began
• to provide before images of changed blocks for consistent and non-blocking reads to take place.

redo is used for 1 purpose

• to roll forward a database after an instance or media failure, to redo transactions that have taken place and commited.

when you perform a transaction, you create both rollack (to undo it) and redo (to do it again if needed)....

On Wed, 23 Jul 1997 20:24:41 -0700, "John C." <john_at_rte-asset.com> wrote:

>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

Thomas Kyte
tkyte_at_us.oracle.com
Oracle Government
Bethesda MD

http://govt.us.oracle.com/ -- downloadable utilities

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Opinions are mine and do not necessarily reflect those of Oracle Corporation Received on Thu Jul 24 1997 - 00:00:00 CDT