As so many of the statements executed against a database generate undo data, managing it is a crucial aspect of database administration. In the past, Oracle used resources called rollback segments
to manage undo data. When a user connected to the database, they were assigned a rollback segment that would serve as storage for the undo data (called rollback data
, back then) generated by that session. Unfortunately, these rollback segments were fairly finite in size, sometimes leading to the infamous
ORA-1555 error, "snapshot too old". This error was raised in situations where queries encountered an undo block that had been overwritten since the query was initiated. In version 9i, Oracle introduced new, more efficient ways to ...