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

相关推荐