why aren't ORA-1555 errors MORE frequent?

I was reading Oracle's NOTE 40689.1 about "delayed block cleanout" (also available in the asktom.oracle.com article "snapshot too old error"). I feel like I'm missing something because I'm left wondering why ORA-1555 errors aren't far MORE frequent than they are. For instance, what keeps ORA-1555 happening in this simple scenario ...

1. A process updates blocks in a table, commits and terminates (delayed block cleanout is used)
2. The blocks in the table are not read again for a long enough time that the pertinent Rollback Segment Headers are overwritten
3. A new process queries those blocks

Since the Rollback Segment Header entries that the modified blocks point to have been overwritten, there's no way for the query to know whether the changes were committed before or after the query started, right? So Oracle must return ORA-1555, right?

Thanks,
Richard Kuhler Received on Mon Nov 18 2002 - 19:03:48 CST