Oracle user_contraints表的constraint_type列中的字母代码代表什么？

select distinct constraint_type from user_constraints;

C
-
C
P
R
U

似乎P表示主键，R表示外键，对吗？什么是U和C？

从12cR1文档中：

C - Check constraint on a table  
P - Primary key  
U - Unique key  
R - Referential integrity  
V - With check option, on a view  
O - With read only, on a view  
H - Hash expression  
F - Constraint that involves a REF column  
S - Supplemental logging

Code Description                Acts On Level
---------------------------------------------
C    Check on a table           Column
O    Read Only on a view        Object
P    Primary Key                Object
R    Referential (Foreign Key)  Column
U    Unique Key                 Column
V    Check Option on a view     Object