Originally posted by grjohnson
I'd be inclined to think you may be getting the error due to a long running query/report on your DB. Therefore I'd have receommended either a). A larger retention time or b). Run the query/report out of the main business hours.

Julian, could you explain why decreasing the retention period would reduce the occurance because it has me a little confused, as Oracle and my own experience tell me to increase. But maybe I'm the one confused

Cheers,
The bigger the retention period, the more commited information needs to be kept in the undo segments. Thus the chance for Oracle to overwrite such commited transactions grows. If such info is overwritten, then ORA-01555 will be raised. That's OK as this is just a warning.