I recently read this in a book about data warehousing:
Source System Rollback SegmentsWhen extracting from a relational source, extracts that take a long time can be problematic. If an extract asks for all records updated in the last 24 hours, the system must locate the appropriate set of records. This means that no user can change the updated_date field while your query is being processed. As transactions flow in during your query, they are queued up in a separate place called a rollback segment, to be applied once your request is finished. If your query takes too long, this queue gets too large and runs out of space. The system then kills your job and processes the transactions that have been queued up. In general, the folks responsible for the transaction system don't like this kind of behavior.
Now to be fair, the book was published in 1998 and is not specifically about Oracle. Does anyone know whether there has ever been a commercial DBMS that worked anything like this, or did they just make it up?