RE: Users reading from rollback segments

Hmmm. I think I CAN predict FUTURE needs of the rollback segments. If there are transactions using the rollback space, users MAY need it. If there are no transactions, then they won't. If I were able to know who is reading from rollback, I would know if shrinking might cause ORA-01555.

Tell me if I'm off on this...

Users will not read from the rollback segment unless they need read-consistent data due to an open transaction against the data they are looking for (thus reading the redo or undo info from rollback). Otherwise they read from the data segments (committed data).

Oracle will not shrink the rollback segment if it contains open transactions.

So, if there are no users reading from rollback and I issue a 'shrink' command, and it works, then the transactions are complete and any user coming in after that will read from the data segments.

If there are no users reading from rollback and I issue a 'shrink' command, and it DOES NOT work, then the transactions are NOT complete and any user coming in after that will read from the rollback segments (the data is still there).

If there ARE users reading from rollback and I issue a 'shrink' command, and it works, then users run the risk of getting ORA-01555 (the data MAY be gone). Which is exactly why I asked my original question ("How do I identify READERS of the rollback segments?") :)

> -----Original Message-----
> From: Jeremiah Wilton [mailto:jwilton_at_speakeasy.net]
> Sent: Tuesday, February 26, 2002 4:24 PM
> To: Multiple recipients of list ORACLE-L
> Subject: Re: Users reading from rollback segments
>
>
> Since you cannot predict who might need to generate consistent reads
> from the RBS in the FUTURE, you cannot predict if you will cause
> ORA-01555 or not by shrinking.
>
> Your best bet is to get rid of people bloating up RBSs by limiting
> their growth, and enforcing the use of smaller transactions. That way
> you won't have to shrink so much.
>
> --
> Jeremiah Wilton
> http://www.speakeasy.net/~jwilton
>
> On Tue, 26 Feb 2002, Glenn Travis wrote:
>
> > Is there a way to tell if anyone is reading from the rollback
> > segments?
> >
> > I would like to manually issue 'alter rollback segment XXX shrink;',
> > but do not want to do so if there are users reading read consistent
> > data from the rollback space (thus giving them the ORA-01555 error).
> >
> > Is there a way to check if the rollback segment is in use first?
> >
> > Can I try to take it offline? Will it fail if there is someone
> > reading from it?
>
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.com
> --
> Author: Jeremiah Wilton
> INET: jwilton_at_speakeasy.net
>
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Glenn Travis
  INET: Glenn.Travis_at_sas.com

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