This is the situation when a query opens a cursor, then loops through fetching, changing, and committing the records on the same table. In this scenerio, very often an ORA-01555 can result. Let's take the following example to explain this:
A cursor was opened at SCN=10. The execution SCN of the query is then marked as SCN=10. Every fetch by that cursor now needs to get the read-consistent data from SCN=10. The user program is now fetching x numbers of records, changing them, and committing them. Let's say they were committed with SCN=20. If a later fetch happens to retrieve a record which is in one of the previously committed blocks, then the fetch will see that the SCN there as 20. Since the fetch has to get the snapshot from SCN=10 it will try to find it in the rollback segments. If it could rollback sufficiently backwards as previously explained, then it could reconstruct the snapshot from SCN=10. If not, then it will result in an ORA-01555 error.
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