Undo stores before image of changes done to the database.
Has three Parameters
In 10g , there is no Manual Option. Undo_supress_errors Parameter deprecated.
v$undostat and v#transaction views are used for statistical information.
Undo_management = Auto is Static Parameter
Undo_tablespace = undotbs1
undo_retention = 900 (sec) these are dynamic.
Only one tablespace can be used.
Undo can be created at the time of the db creation and also after db creation can assign another tbs.
Error- ORA-1555 Snapshot too Old - Occurs if the tablespace or retention is to be tuned.
- with a too-small undo_retention, even with a large undo tables.
- If you have lots of updates, long running SQL and too small UNDO, the ORA-01555 error will appear
#Undo Remedies:
0 - Re-schedule long-running queries when the system has less DML load.
1 - Increasing the size of your rollback segment (undo) size. The ORA-01555 snapshot too old also relates to your setting for automatic undo retention.
2 - Don't fetch between commits.