Undo data is generated by all DML commands. The length of time that the undo data
is retained for is, at a minimum, the length of the transaction. It will, however,
frequently be necessary to store undo data for some considerable time after the
transaction has finished. This is to satisfy the need for consistent reads: the C of the
ACID test. This requirement is determined by the length of the longest-running
queries executed against the database. In order to prevent the “ORA-1555: snapshot
too old” error, the undo tablespace must be large enough to retain enough undo data
to prevent this. If you intend to make use of the Flashback Query capability of the
database (which will be detailed in Chapter 29), it may be necessary to retain undo
data for considerably longer.
The algorithm to size the undo tablespace is based on the rate of undo generation
per second and the number of seconds of data it is necessary to store to meet the
requirements of long-running queries, and possibly the use of Flashback Query. The
Undo Advisor shows a graph of the necessary size of the undo tablespace for various
undo retention times.   _{Your Comment}