Re: What is the difference with Redo log, Archivelog, Undo ?

On Mar 20, 6:43 am, "Charles Hooper" <hooperc2..._at_yahoo.com> wrote:
> On Mar 20, 6:22 am, "robertchen..._at_gmail.com"
>
> <robertchen..._at_gmail.com> wrote:
> > What is the difference with Redo log, Archivelog, Undo ?
>
> > I think Redo log is the same with archivelog?
>
> The most complete answers can be found here:
> http://www.oracle.com/pls/db102/search?word=redo+undo&partno=
>
> In short, the redo log contains undo information also. When a redo
> log becomes full, it is eligible to become an archived redo log, and
> stored in a different location (if the database is set up IN
> ARCHIVELOG MODE. Undo is used to roll back transactions and provide
> read consistency, while redo is used to roll forward transactions in
> the event of problems.
>
> SELECT
> LOG_MODE
> FROM
> V$DATABASE;
>
> Charles Hooper
> PC Support Specialist
> K&M Machine-Fabricating, Inc.

To add to what Charles posted:
Redo and undo are explained in the Concepts manual. Redo is for database forward recovery
Undo is for transaction rollback and read consistency

Redo and undo are used together during crash recovery and database media recovery operations. During recovery operations you first restore the data files, you roll forward using the archived redo logs and the online redo log, and then you rollback incomplete transactions using the recovered, rolled forward undo segments.

See the Backup and Recovery manual for a more detailed explanation.

HTH -- Mark D Powell -- Received on Tue Mar 20 2007 - 09:31:35 CDT