Undo Retention

Undo Retention
Oracle Database automatically ensures that undo data that is in use by an active
transaction is never overwritten until that transaction has been committed. After the
transaction has been committed, the space occupied by that undo data can be reused,
or overwritten. In this case, that undo data could be overwritten if space in the undo
tablespace becomes scarce.
Even after a transaction has been committed, it is useful to retain (not overwrite) its
undo data, to ensure the success of Oracle Flashback features and for read consistency
for long-running queries. To this end, the database maintains and automatically tunes
an undo retention period. Committed undo data whose age is less than the undo
retention period is retained for use by queries or Oracle Flashback operations.
... zobacz całą notatkę