Oracle 11G 虚拟列 Virtual Column使用

原文链接：http://blog.csdn.net/zhongweijian/article/details/9008067

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