1 UNDO TABLESPACE For more information about setting automatic undo management, see Oracle documentation.

1.1 UNDO retention period

UNDO_RETENTION=In seconds the time of undo information retained.
Using rollback segments, the rollback segments need to large enough to contain the undo required to reconstruct the view of the object at the time you establish for the flashback query.
If you get an ORA-01555 snapshot too old: rollback segment number string with name "string" too small
Means that you must increase the setting of UNDO_RETENTION or otherwise use larger rollback segments.

1.2 Retention Guarantee

To guarantee the success of long-running queries or Oracle Flashback operations, you can enable retention guarantee. If retention guarantee is enabled, the specified minimum undo retention is guaranteed; the database never overwrites unexpired undo data even if it means that transactions fail due to lack of space in the undo tablespace. ("Unexpired" undo data is old undo data whose age is still less than the current retention period.) If retention guarantee is not enabled, the database can overwrite unexpired undo when space is low, thus lowering the undo retention for the system. This action can potentially cause some queries to fail with the snapshot too old error message. This option is disabled by default.
WARNING:
Enabling retention guarantee can cause multiple DML operations to fail. Use with caution.
You enable retention guarantee by specifying the RETENTION GUARANTEE clause for the undo tablespace when you create it with either the CREATE DATABASE or CREATE UNDO TABLESPACE statement. Or, you can later specify this clause in an ALTER TABLESPACE statement. You disable retention guarantee with the RETENTION NOGUARANTEE clause.