Re: Question on REDO & SCN

"Pradeep" <agarwalp_at_eeism.com> wrote in message news:1132845637.997279.213440_at_o13g2000cwo.googlegroups.com...
> Thanks for the answer Jonathan
>
>>> The redo is the bit that describes the change
>>>to the undo segment header block that marks
>>>the transaction as complete
>
> Isn't this the SCN?
>

No, the SCN is just a number being counted up somewhere in memory.

To start a transaction you acquire a row (aka slot) in the transaction table that exists in each undo segment header block. This is the thing that your TX lock is locking. You update this row to show that your transaction is running, and to show the start SCN and a couple of other bits of information.

To end a transaction, you update the row with the commit SCN, and flag the row to show that it is free. It is this change that is 'just another block change' and therefore described by a bit of redo, which the manuals (incorrectly) call the 'commit record'.

-- 
Regards

Jonathan Lewis

http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ

http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
Cost Based Oracle: Fundamentals

http://www.jlcomp.demon.co.uk/appearances.html
Public Appearances - schedule updated 4th Nov 2005