Oracle 11g 虚拟列 Virtual Column介绍

Oracle 11G 虚拟列 Virtual Column

Oracle 11G 在表中引入了虚拟列，虚拟列是一个表达式，在运行时计算，不存储在数据库中，不能更新虚拟列的值。

定义一个虚拟列的语法：

	column_name [datatype] [GENERATED ALWAYS] AS [expression] [VIRTUAL]

1.虚拟列可以用在select，update，delete语句的where条件中，但是不能用于DML语句
2.可以基于虚拟列来做分区
3. 可以在虚拟列上建索引，oracle的函数索引就类似。
4. 可以在虚拟列上建约束

创建一个带虚拟列的表：

	CREATE TABLE EMPLOYEE
	(
		empl_id        NUMBER,
		empl_nm        VARCHAR2(50),
		monthly_sal    NUMBER(10,2),
		bonus          NUMBER(10,2),
		total_sal      NUMBER(10,2) GENERATED ALWAYS AS (monthly_sal*12 + bonus)
	);

 total_sal就是一个虚拟列

查看下表定义内容：

	SELECT column_name, data_type, data_length, data_default, virtual_column
	  FROM user_tab_cols
	 WHERE table_name = ‘EMPLOYEE‘;

	COLUMN_NAME | DATA_TYPE | DATA_LENGTH | DATA_DEFAULT             | VIRTUAL_COLUMN
	EMPL_ID     | NUMBER    | 22          | null                     | NO
	EMPL_NM     | VARCHAR2  | 50          | null                     | NO
	MONTHLY_SAL | NUMBER    | 22          | null                     | NO
	BONUS       | NUMBER    | 22          | null                     | NO
	TOTAL_SAL   | NUMBER    | 22          | "MONTHLY_SAL"*12+"BONUS" | YES        

virtual_column是yes，表示是虚拟列，我们也可以在建表语句加上VIRTUAL，显示声明为虚拟列

	DROP TABLE EMPLOYEE PURGE;

	CREATE OR REPLACE FUNCTION get_empl_total_sal ( p_monthly_sal   NUMBER,
													p_bonus         NUMBER)
	   RETURN NUMBER
	DETERMINISTIC
	IS
	BEGIN
	   RETURN p_monthly_sal * 12 + p_bonus;
	END;

	CREATE TABLE EMPLOYEE
	(empl_id     NUMBER,
	 empl_nm     VARCHAR2(50),
	 monthly_sal NUMBER(10,2),
	 bonus       NUMBER(10,2),
	 total_sal   NUMBER(10,2) AS (get_empl_total_sal(monthly_sal, bonus)) VIRTUAL
	);

我们可以在虚拟列上创建索引

	CREATE INDEX idx_total_sal ON employee(total_sal);

	SELECT index_name, index_type
	  FROM user_indexes
	 WHERE table_name = ‘EMPLOYEE‘;

	INDEX_NAME     INDEX_TYPE
	IDX_TOTAL_SAL  FUNCTION-BASED NORMAL

这个函数也是表定义内容

	DROP FUNCTION get_empl_total_sal;

	SELECT * FROM employee;
	*
	Error at line 0
	ORA-00904: "schema"."GET_EMPL_TOTAL_SAL": invalid identifier

可以添加一个虚拟列

	DROP TABLE EMPLOYEE PURGE;

	CREATE TABLE EMPLOYEE
	(empl_id     NUMBER,
	 empl_nm     VARCHAR2(50),
	 monthly_sal NUMBER(10,2),
	 bonus       NUMBER(10,2)
	);

	ALTER TABLE EMPLOYEE ADD (total_sal AS (monthly_sal * 12 + bonus));

这个新列的类型时不确定的，是根据表达式来推断的。

	INSERT INTO employee (empl_id, empl_nm, monthly_sal, bonus)
	   WITH DATA AS
			(SELECT 100 empl_id, ‘AAA‘ empl_nm, 20000 monthly_sal, 3000 bonus
			   FROM DUAL
			 UNION
			 SELECT 200, ‘BBB‘, 12000, 2000
			   FROM DUAL
			 UNION
			 SELECT 300, ‘CCC‘, 32100, 1000
			   FROM DUAL
			 UNION
			 SELECT 400, ‘DDD‘, 24300, 5000
			   FROM DUAL
			 UNION
			 SELECT 500, ‘EEE‘, 12300, 8000
			   FROM DUAL)
	   SELECT *
		 FROM DATA;

	SELECT * FROM employee;

	EMPL_ID | EMPL_NM | MONTHLY_SAL | BONUS | TOTAL_SAL
	100     | AAA     | 20000       | 3000  | 243000
	200     | BBB     | 12000       | 2000  | 146000
	300     | CCC     | 32100       | 1000  | 386200
	400     | DDD     | 24300       | 5000  | 296600
	500     | EEE     | 12300       | 8000  | 155600

在虚拟列上执行update语句是不允许的

	UPDATE employee
	   SET total_sal = 2000;

	ORA-54017: UPDATE operation disallowed on virtual columns

能够从虚拟列上收集表的统计信息

	EXEC DBMS_STATS.GATHER_TABLE_STATS(user, ‘EMPLOYEE‘);

	SELECT column_name, num_distinct,
		   display_raw (low_value, data_type)  low_value,
		   display_raw (high_value, data_type) high_value
	  FROM dba_tab_cols
	 WHERE table_name = ‘EMPLOYEE‘;

	COLUMN_NAME | NUM_DISTINCT | LOW_VALUE | HIGH_VALUE
	TOTAL_SAL   | 5            | 146000    | 386200
	BONUS       | 5            | 1000      | 8000
	MONTHLY_SAL | 5            | 12000     | 32100
	EMPL_NM     | 5            | AAA       | EEE
	EMPL_ID     | 5            | 100       | 500

基于虚拟列的分区

	DROP TABLE EMPLOYEE PURGE;

	CREATE TABLE employee
	(empl_id     NUMBER,
	 empl_nm     VARCHAR2(50),
	 monthly_sal NUMBER(10,2),
	 bonus       NUMBER(10,2),
	 total_sal   NUMBER(10,2) AS (monthly_sal*12 + bonus)
	)
	PARTITION BY RANGE (total_sal)
		(PARTITION sal_200000 VALUES LESS THAN (200000),
		 PARTITION sal_400000 VALUES LESS THAN (400000),
		 PARTITION sal_600000 VALUES LESS THAN (600000),
		 PARTITION sal_800000 VALUES LESS THAN (800000),
		 PARTITION sal_default VALUES LESS THAN (MAXVALUE));

	INSERT INTO employee (empl_id, empl_nm, monthly_sal, bonus)
	   WITH DATA AS
			(SELECT 100 empl_id, ‘AAA‘ empl_nm, 20000 monthly_sal, 3000 bonus
			   FROM DUAL
			 UNION
			 SELECT 200, ‘BBB‘, 12000, 2000
			   FROM DUAL
			 UNION
			 SELECT 300, ‘CCC‘, 32100, 1000
			   FROM DUAL
			 UNION
			 SELECT 400, ‘DDD‘, 24300, 5000
			   FROM DUAL
			 UNION
			 SELECT 500, ‘EEE‘, 12300, 8000
			   FROM DUAL)
	   SELECT *
		 FROM DATA;

	EXEC DBMS_STATS.GATHER_TABLE_STATS(user,‘EMPLOYEE‘,granularity => ‘PARTITION‘);

	SELECT   table_name, partition_name, num_rows
		FROM user_tab_partitions
	   WHERE table_name = ‘EMPLOYEE‘
	ORDER BY partition_name;

	TABLE_NAME | PARTITION_NAME | NUM_ROWS
	EMPLOYEE   | SAL_200000     | 2
	EMPLOYEE   | SAL_400000     | 3
	EMPLOYEE   | SAL_600000     | 0
	EMPLOYEE   | SAL_800000     | 0
	EMPLOYEE   | SAL_DEFAULT    | 0

在分区情况下，不能更新虚拟列引用的列

	UPDATE employee
	   SET monthly_sal = 30000
	 WHERE empl_id = 500;

	ORA-14402: updating partition key column would cause a partition change

如果在分区情况能够更新，则需要设置ENABLE ROW MOVEMENT

	ALTER TABLE employee ENABLE ROW MOVEMENT;

	UPDATE employee
	   SET monthly_sal = 80000
	 WHERE empl_id = 500;

	1 row updated.

不能基于函数表达式的虚拟列上的分区

	CREATE TABLE employee_new
	(empl_id     NUMBER,
	 empl_nm     VARCHAR2(50),
	 monthly_sal NUMBER(10,2),
	 bonus       NUMBER(10,2),
	 total_sal   NUMBER(10,2) AS (get_empl_total_sal(monthly_sal, bonus))
	)
	PARTITION BY RANGE (total_sal)
		(PARTITION sal_200000 VALUES LESS THAN (200000),
		 PARTITION sal_400000 VALUES LESS THAN (400000),
		 PARTITION sal_600000 VALUES LESS THAN (600000),
		 PARTITION sal_800000 VALUES LESS THAN (800000),
		 PARTITION sal_default VALUES LESS THAN (MAXVALUE));

	ORA-54021: Cannot use PL/SQL expressions in partitioning or subpartitioning columns