Re: Question on REDO & SCN

"Pradeep" <agarwalp_at_eeism.com> wrote in message news:1132843965.534875.93730_at_g47g2000cwa.googlegroups.com...
>I am presently reading Expert one-on-one oracle, but have a few
> questions:
>
> 1. After a DML statement all the required redo is generated. But if i
> commit there is a small amount of redo generated. What is this redo ?
> (i think that this redo is is generated after the undo is updated with
> the SCN on COMMIT)
>

    The redo is the bit that describes the change     to the undo segment header block that marks     the transaction as complete (and possibly puts     the current undo block into the free block pool)     When a normal data block is touched on commit,     (and in that case it isn't a proper cleanout) the redo     for that is not generated. Proper cleanout is generated     by the next transaction that changes the block - or by     a delayed block cleanout.

> 2. delay block cleanout happens by getting the status of the
> transaction from the UNDO. In case the undo is overwritten, then it
> gives an error. My questions is why does it not take the SCN for that
> transaction from the redo (if not avaible in undo) and do block
> cleanout?
>

    Because NOTHING reads the online redo log     under normal circumstances. It is important that     the redo log be written as rapidly as possible, so     it is effectively a 'write only' stream.

-- 
Regards

Jonathan Lewis

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