Re: ORA-01555; does update/select sequence matter?

ctcgag_at_hotmail.com wrote in message news:<20031113000943.508> >
> > 1. Session 1 modifies 100 blocks in a table at time T1. 100 blocks
> > copied to rollback segment with SCN marked as, say, SCN1.
> > 2. Session 2 comes in at time T2 and select one of the blocks, let's
> > say block1. Since the data is being modified by session 1, session 2
> > goes to the rollback for the block. Let's say the current SCN is SCN2.
> > 3. At time T3 session 1 commits. The rollback is now up for grab.
> > Let's say it is overwritten before session 2 finishes the long runnig
> > query.
> > 4. At time T4 session 2 ask for another data block, say block100.
> >
> > Here is the critical point. What happens when session 2 ask for
> > block100 at T4? It looks up block100 and notice the data in the block
> > has been committed and the SCN is SCN1 which is older than SCN2, the
> > starting SCN of the query.
>
> No, the SCN of the committed data is the SCN of the commit (T3),
> not the SCN of when the updating transaction started (T1). You can't
> commit retroactively. So session 2 does need to rollback the committed
> data, but can't.
>
> Xho

Thanks, I think this is where I did not have a good understanding. I thought the SCN in generated when the DML is issued and stick with the data blocks until the transaction ends.

John Received on Thu Nov 13 2003 - 09:31:22 CST