OCP 1Z0 052 30

30. In which situation may the UNDO_RETENTION parameter be ignored, even if it is set to a value? 
A.when the data file of the undo tablespace is autoextensible
B.when there are more than one undo tablespaces available in the database
C.when the undo tablespace is of a fixed size and retention guarantee is not enabled
D.when the undo tablespace is autoextensible and retention guarantee is not enabled
Answer: C

For AUTOEXTEND undo tablespaces, the system retains undo for at least the time specified in this parameter, and automatically tunes the undo retention period to satisfy the undo requirements of the queries. 

For fixed- size undo tablespaces, the system automatically tunes for the maximum possible undo retention period, based on undo tablespace size and usage history, and ignores UNDO_RETENTION unless retention guarantee is enabled.

1、设置为自增时，将最大限度的保留undo，以供查询所需要

2、非自增时，同时最大限度的保留。但因空间有限，当空间不够时就没个么好说的了，先拿来用吧，除非设定guarantee 。