Re: Redo Logs and Rollback Segments Relationships ?

Jonathan B. Joseph wrote:

> I'm an Oracle newbie and I was recently reviewing a database creation
> script
> and noticed that there were entries for creating both redo logs and
> rollback segments.
> Since I thought that rollback segments were the physical components of
> the redo logs,
> I need to understand how they are related. So I have the following
> questions:
>
> Are rollback segments related to redo logs ?
>
> When I execute an update, insert , or delete statement , does it
> get written to the redo log or rollback segment for the tablespace
> that I'm working in ? (ie., what happens to the redo log and/or
> rollback segment ?)
>
> Do rollback segements compose the physical parts of a redo log ?
>
> And what happens to the redo log and/or rollback segment, when I
> issue a commit ? Do do all the inserts, updates, deletes get removed
> from the rollback segment and redo log and the inserts, updates,
> deletes get written to the tablespace of the database that I'm working
> in?
>
> When I do a rollback , does the rollback segment and/or redo log
> get erased (and the inserts, updates etc. not get applied to the
> database) ?
>
> Thank you very much for any help,
> Jonathan Joseph
> ----------------
> jon.joseph_at_jbjnet.com

A complete answer to your questions would fill a book ! In short -
Rollback segments are used to ensure read consistency while redo logs are used to recover a database. When an update transaction changes data, the before-image is stored in the rollback segments for use by transactions that may have begun before this update transaction began. Both before- and after-images of the updated data are stored in redo logs for recovery purposes.
Pick up a good book to get more in-depth understanding of these concepts.

--
hope this helps !
Srini

(to reply, change "nospam" to "chavali" in my email address.  :-)  )