Re: Rollback Segment: Read Consistency Errors

Think about it. Maxextents is simply an upper bound to which the rollback segment can grow *if it needs to*. The whole point of snapshots too old is that the rollback segment has no need to grow bigger -it can happily reuse existing space used by prior transactions, because those transactions have long since been committed or rolled back. Unfortunately, in doing so it overwrites the rollback needed to provide a read-consistent image.

The only thing that can prevent 1555s is if the rollback segment is ALREADY bigger, not whether it is permitted, at some unspecified point in the future, to become bigger, which is all that a new MAXEXTENTS clause would do.

Regards
HJR "Dino Hsu" <dino1_at_ms1.hinet.net> wrote in message news:042cntgp6ejdnm06am6gmt72t3qib8fkv5_at_4ax.com...
> Dear all,
>
> Quoting from a book:
> "Troubleshooting Rollback Segment Problems:
> Read Consistency Errors:
> Recall that rollback segments allow transaction-level read consistency
> for all statements running on the database. A long-running query in
> one session that starts before a change on that data is committed by
> another session will see the data as it existed pre-commit. If Oracle
> cannot provide a read-consistent view of data, the following error
> will ensue:
>
> ORA-01555 - snapshot too old (rollback segment too small)
>
> The solution to the problem is to re-create the rollback segments with
> a higher minimum number of extents, larger extents, or a higher
> optimal rollback segment size. You cannot simply increase maxextents
> to resolve this problem."
>
> If the maxextents is bigger, the max rollback segment size is bigger,
> so it takes longer to get this snapshot too old error. Does my
> rationale hold? Thanks in advance.
>
> Dino
>
Received on Sun Aug 12 2001 - 19:25:28 CDT