Invisible Columns

使用select * from ，desc 等看不到该列，

DROP TABLE tab1 PURGE;

CREATE TABLE tab1 (
  id          NUMBER,
  description VARCHAR2(50) INVISIBLE
);

DESC tab1;
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID                                                 NUMBER

SQL>

INSERT INTO tab1 VALUES (1);
COMMIT;

SELECT * FROM tab1;

        ID
----------
         1

SQL>

 

在明确的指定它的名字进行查询的时候仍然是可用的

INSERT INTO tab1 (id, description) VALUES (2, ‘TWO‘);
COMMIT;

SELECT id, description
FROM   tab1;

        ID DESCRIPTION
---------- --------------------------------------------------
         1
         2 TWO

SQL>

 

下面关于隐含列的几项内容

• 虚拟列可以设成invisible
• 一个表可以对隐含列就行分区，
• 外部表，表簇，临时表中不能包含隐含列
• 用户自定义的类型不能包含隐含属性
• You can not make system generated hidden columns visible.

Invisible Columns and Column Ordering

隐含列不分配列的顺序（colum_id），如果一个可见的列设为不可见，那么这个列将放到最后一列

DROP TABLE tab1 PURGE;

CREATE TABLE tab1 (
  a NUMBER,
  b NUMBER,
  c NUMBER INVISIBLE
);

COLUMN column_name FORMAT A15

SELECT column_id,
       column_name,
       hidden_column
FROM   user_tab_cols
WHERE  table_name = ‘TAB1‘
ORDER BY column_id;

 COLUMN_ID COLUMN_NAME     HID
---------- --------------- ---
         1 A               NO
         2 B               NO
           C               YES

SQL> 

ALTER TABLE tab1 MODIFY b INVISIBLE;
ALTER TABLE tab1 MODIFY c VISIBLE;

SELECT column_id,
       column_name,
       hidden_column
FROM   user_tab_cols
WHERE  table_name = ‘TAB1‘
ORDER BY column_id;

 COLUMN_ID COLUMN_NAME     HID
---------- --------------- ---
         1 A               NO
         2 C               NO
           B               YES

SQL>

ALTER TABLE tab1 MODIFY b VISIBLE;

SELECT column_id,
       column_name,
       hidden_column
FROM   user_tab_cols
WHERE  table_name = ‘TAB1‘
ORDER BY column_id;

 COLUMN_ID COLUMN_NAME     HID
---------- --------------- ---
         1 A               NO
         2 C               NO
         3 B               NO

SQL>

 

强制类型Invisible Columns

隐含列的强制/可选等属性仍然是有效的，如下例：（隐含列的not null属性仍然是生效的）

DROP TABLE tab1 PURGE;

CREATE TABLE tab1 (
  id          NUMBER NOT NULL,
  description VARCHAR2(50) NOT NULL,
  created_date DATE INVISIBLE NOT NULL
);

COLUMN column_name FORMAT A20

SELECT column_id,
       column_name,
       nullable,
       hidden_column
FROM   user_tab_cols
WHERE  table_name = ‘TAB1‘
ORDER BY column_id;

 COLUMN_ID COLUMN_NAME          N HID
---------- -------------------- - ---
         1 ID                   N NO
         2 DESCRIPTION          N NO
           CREATED_DATE         N YES

SQL>

INSERT INTO tab1 VALUES (1, ‘ONE‘);
*
ERROR at line 1:
ORA-01400: cannot insert NULL into ("TEST"."TAB1"."CREATED_DATE")

SQL>

ALTER TABLE tab1 MODIFY created_date NULL;
INSERT INTO tab1 VALUES (1, ‘ONE‘);

1 row created.

SQL>

oracle 12c中的隐含列