Re: Redo log and rollback segment?

A copy of this was sent to Doug Cowles <dcowles_at_nospambigfoot.com> (if that email address didn't require changing) On Sun, 19 Sep 1999 02:49:33 -0400, you wrote:

>I think you explained in a thread a few months ago that redo actually contains both
>before and after images. Is that correct?
>

rollback is logged (part of the necessities for instance recovery).

therefore, the AFTER IMAGE (redo) of rollback (which happens to be the BEFORE IMAGE (undo) of the object being modified) is in the redo logs.

so yes, in a round about way -- redo contains before and after images of the modified object since it contains the AFTER images of the modified object and it contains the AFTER images of the rollback generated during the modification of that object.

felt that such a comment would even further confuse the issue :)

>Thomas Kyte wrote:
>
>> A copy of this was sent to jonathan_at_ms35.url.com.tw (Trister Yang)
>> (if that email address didn't require changing)
>> On 3 Sep 1999 02:10:10 GMT, you wrote:
>>
>> >I always so confuse about what's different between redo log and
>> >rollback segment ?
>> >
>> >I will appreciate for your answer!
>> >
>> >Thanks!
>>
>> REDO is 'redo' -- it can redo a transaction. It is after images of changes. It
>> is predominantly used for RECOVERY after a crash or media failure.
>>
>> ROLLBACK is 'undo' -- it can undo a transaction. It is before images of
>> changes. It is predominantly used to rollback a transaction after an online
>> error or to provide read consistent, nonblocking views of data.
>>
>> --
>> See http://govt.us.oracle.com/~tkyte/ for my columns 'Digging-in to Oracle8i'...
>> Current article is "Part I of V, Autonomous Transactions" updated June 21'st
>>
>> Thomas Kyte tkyte_at_us.oracle.com
>> Oracle Service Industries Reston, VA USA
>>
>> Opinions are mine and do not necessarily reflect those of Oracle Corporation
>

--
See http://govt.us.oracle.com/~tkyte/ for my columns 'Digging-in to Oracle8i'... Current article is "Part I of V, Autonomous Transactions" updated June 21'st  

Thomas Kyte                   tkyte_at_us.oracle.com
Oracle Service Industries     Reston, VA   USA

Opinions are mine and do not necessarily reflect those of Oracle Corporation Received on Sun Sep 19 1999 - 08:25:11 CDT