DB是12CR1

SQL> create table tb_zxp(id integer primary key,uu blob);

Table created.

SQL> select COLUMN_NAME,retention from user_lobs where table_name='TB_ZXP' order by 1;

COLUMN_NAME RETENTION
------------------------------ ----------
UU

SQL> alter table tb_zxp modify lob(uu) (retention);

Table altered.

SQL> show parameter undo_retention;

NAME TYPE VALUE
------------------------------------ --------------------------------- ------------------------------
undo_retention integer 18000

SQL> select COLUMN_NAME,retention from user_lobs where table_name='TB_ZXP' order by 1;

COLUMN_NAME RETENTION
------------------------------ ----------
UU

疑问一：建表时，既然UNDO_RETENTION有值了，为何该LOB字段UU的RETENTION还是NULL？ 疑问二：显式地又alter table modify lob一次了，为何RETENTION还是为NULL？