DBA_TABLESPACES中的RETENTION列说明：
Undo tablespace retention:
GUARANTEE---Tablespace is an undo tablespace with RETENTION specified as GUARANTEE
A RETENTION value of GUARANTEE indicates that unexpired undo in all undo segments in the undo tablespace should be retained even if it means that forward going operations that need to generate undo in those segments fail.
NOGUARANTEE—Tablespace is an undo tablespace with RETENTION specified as NOGUARANTEE
NOT APPLY—Tablespace is not an undo tablespace