In the previous couple of chapters we discussed several undo segment topics. We’ve seen how they are used during recovery, how they interact with the redo logs, and how they are used for consistent, nonblocking reads of data. In this chapter, we’ll look at the most frequently raised issues with undo segments.
In this chapter the bulk of our time will be spent on the infamous ORA-01555: snapshot too old error, as this single issue causes more confusion than any other topic in the entire set of database topics. Before we do this, however, we’ll investigate one other undo-related issue: the question of what type of DML ...