您好,登錄后才能下訂單哦!
Oracle 11g新特性之--虛擬列(Virtual Column)
Oracle 11G虛擬列Virtual Column介紹
在老的 Oracle 版本,當我們需要使用表達式或者一些計算公式時,我們會創(chuàng)建數(shù)據(jù)庫視圖,如果我們需要在這個視圖上使用索引,我們會創(chuàng)建基于函數(shù)的索引。
我們從Oracle官方文檔中,找到下面對于虛擬列技術(shù)的描述。
“Tables can also include virtual columns. A virtual column is like any other table column, except that its value is derived by evaluating an expression. The expression can include columns from the same table, constants, SQL functions, and user-defined PL/SQL functions. You cannot explicitly write to a virtual column.”
Oracle 11G 在表中引入了虛擬列,虛擬列是一個表達式,在運行時計算,不存儲在數(shù)據(jù)庫中,不能更新虛擬列的值。
定義一個虛擬列的語法:
column_name [datatype] [GENERATED ALWAYS] AS [expression] [VIRTUAL]
1.虛擬列可以用在select,update,delete語句的where條件中,但是不能用于DML語句
2.可以基于虛擬列來做分區(qū)
3. 可以在虛擬列上建索引,oracle的函數(shù)索引就類似。
4. 可以在虛擬列上建約束
案例:
1、創(chuàng)建一個帶虛擬列的表:
14:51:28 SCOTT@ test1 >CREATE TABLE EMP3 14:51:51 2 ( 14:51:51 3 EMPNO NUMBER(6), 14:51:51 4 SAL NUMBER(8,2), 14:51:51 5 COMM NUMBER(8,2), 14:51:51 6 SAL_PACK GENERATED ALWAYS AS ( SAL + NVL(COMM,0) ) VIRTUAL 14:51:51 7 ) Table created.
2、查看虛擬列屬性
14:56:10 SCOTT@ test1 >COL TABLE_NAME FOR A10 14:56:19 SCOTT@ test1 >COL COLUMN_NAME FOR A20 14:56:27 SCOTT@ test1 >COL DATA_TYPE FOR A20 14:56:34 SCOTT@ test1 >COL DATA_DEFAULT FOR A20 14:56:48 SCOTT@ test1 >R 1 select table_name,COLUMN_NAME,data_type,data_default,VIRTUAL_COLUMN from user_tab_cols 2* where table_name='EMP3' TABLE_NAME COLUMN_NAME DATA_TYPE DATA_DEFAULT VIR ---------- -------------------- -------------------- -------------------- --- EMP3 SAL_PACK NUMBER "SAL"+NVL("COMM",0) YES EMP3 COMM NUMBER NO EMP3 SAL NUMBER NO EMP3 EMPNO NUMBER NO
上述建的虛擬列 SAL_PACK 是由一個簡單的表達式創(chuàng)建的,使用的關(guān)鍵字有 VIRTUAL(不過這個關(guān)鍵字是可選的),該字段的值是由 COMM 這個字段通過表達式計算而來的。
在Table上添加虛擬列:
15:44:12 SCOTT@ test1 >alter table emp3 add (sal_total as (sal*12+comm) virtual); Table altered. 15:49:11 SCOTT@ test1 >desc emp3; Name Null? Type ----------------------------------------------------------------- -------- -------------------------------------------- EMPNO NUMBER(6) SAL NUMBER(8,2) COMM NUMBER(8,2) SAL_PACK NOT NULL NUMBER SAL_TOTAL NUMBER 15:49:16 SCOTT@ test1 >select * from emp3; EMPNO SAL COMM SAL_PACK SAL_TOTAL ---------- ---------- ---------- ---------- ---------- 10 1500 500 2000 18500 20 3000 500 3500 36500 30 4000 500 4500 48500 40 6000 500 6500 72500 15:51:00 SCOTT@ test1 >select table_name,COLUMN_NAME,data_type,data_default,VIRTUAL_COLUMN from user_tab_cols 15:51:27 2 where table_name='EMP3'; TABLE_NAME COLUMN_NAME DATA_TYPE DATA_DEFAULT VIR ---------- -------------------- -------------------- -------------------- --- EMP3 SAL_TOTAL NUMBER "SAL"*12+"COMM" YES EMP3 SAL_PACK NUMBER "SAL"+NVL("COMM",0) YES EMP3 COMM NUMBER NO EMP3 SAL NUMBER NO EMP3 EMPNO NUMBER NO
在虛擬列中使用函數(shù):
15:51:37 SCOTT@ test1 >CREATE OR REPLACE FUNCTION sum_sal (in_num1 NUMBER, in_num2 NUMBER) 15:57:17 2 RETURN NUMBER DETERMINISTIC 15:57:17 3 AS 15:57:17 4 BEGIN 15:57:17 5 RETURN in_num1 + in_num2; 15:57:18 6 END; 15:57:19 7 / Function created. 15:57:21 SCOTT@ test1 >alter table emp3 add ( sal_comm as (sum_sal(sal,comm)) virtual); Table altered. 16:00:03 SCOTT@ test1 >desc emp3 Name Null? Type ----------------------------------------------------------------- -------- -------------------------------------------- EMPNO NUMBER(6) SAL NUMBER(8,2) COMM NUMBER(8,2) SAL_PACK NOT NULL NUMBER SAL_TOTAL NUMBER SAL_COMM NUMBER 16:00:07 SCOTT@ test1 >select * from emp3; EMPNO SAL COMM SAL_PACK SAL_TOTAL SAL_COMM ---------- ---------- ---------- ---------- ---------- ---------- 10 1500 500 2000 18500 2000 20 3000 500 3500 36500 3500 30 4000 500 4500 48500 4500 40 6000 500 6500 72500 6500
虛擬列的值是不存儲在磁盤的,它們是在查詢時根據(jù)定義的表達式臨時計算的。
3、對虛擬列的操作
Insert 操作:
我們不能往虛擬列中插入數(shù)據(jù): 15:01:52 SCOTT@ test1 >insert into emp3 values (10,1500,500,2000); insert into emp3 values (10,1500,500,2000) * ERROR at line 1: ORA-54013: INSERT operation disallowed on virtual columns 也不能隱式的添加數(shù)據(jù)到虛擬列: 15:02:16 SCOTT@ test1 >insert into emp3 values (10,1500,500); insert into emp3 values (10,1500,500) * ERROR at line 1: ORA-00947: not enough values 虛擬列的數(shù)據(jù)會自動計算生成 15:07:16 SCOTT@ test1 >insert into emp3(empno,sal,comm) values (10,1500,500); 1 row created. 15:07:29 SCOTT@ test1 >select * from emp3; EMPNO SAL COMM SAL_PACK ---------- ---------- ---------- ---------- 10 1500 500 2000
對虛擬列不能做update操作:
15:18:45 SCOTT@ test1 >update emp3 set sal_pack=3000; update emp3 set sal_pack=3000 * ERROR at line 1: ORA-54017: UPDATE operation disallowed on virtual columns
在虛擬列上創(chuàng)建索引和約束:
15:19:07 SCOTT@ test1 >create index emp3_val_ind on emp3(sal_pack) tablespace indx; Index created. 15:21:20 SCOTT@ test1 >select table_name,index_name,INDEX_TYPE from user_indexes 15:22:11 2 where table_name='EMP3'; TABLE_NAME INDEX_NAME INDEX_TYPE ---------- ------------------------------ --------------------------- EMP3 EMP3_VAL_IND FUNCTION-BASED NORMAL 15:22:18 SCOTT@ test1 >drop index EMP3_VAL_IND; Index dropped. 15:24:37 SCOTT@ test1 >alter table emp3 add constraint pk_emp3 primary key (sal_pack); Table altered. 15:25:22 SCOTT@ test1 >select table_name,index_name,INDEX_TYPE from user_indexes 15:25:34 2 where table_name='EMP3'; TABLE_NAME INDEX_NAME INDEX_TYPE ---------- ------------------------------ --------------------------- EMP3 PK_EMP3 FUNCTION-BASED NORMAL
在虛擬列上建立分區(qū)表:
15:41:43 SCOTT@ test1 >CREATE TABLE EMP3_part 15:41:46 2 ( 15:41:46 3 EMPNO NUMBER(6), 15:41:46 4 SAL NUMBER(8,2), 15:41:46 5 COMM NUMBER(8,2), 15:41:46 6 SAL_PACK GENERATED ALWAYS AS ( SAL + NVL(COMM,0) ) VIRTUAL 15:41:46 7 ) 15:41:46 8 PARTITION BY range (sal_pack) 15:41:46 9 (PARTITION sal_2000 VALUES LESS THAN (2000), 15:41:46 10 PARTITION sal_4000 VALUES LESS THAN (4000), 15:41:46 11 PARTITION sal_6000 VALUES LESS THAN (6000), 15:41:46 12 PARTITION sal_8000 VALUES LESS THAN (8000), 15:41:46 13 PARTITION sal_default VALUES LESS THAN (MAXVALUE)); Table created. 15:42:33 SCOTT@ test1 >insert into emp3_part(empno,sal,comm) select empno,sal,comm from emp3; 4 rows created. 15:43:33 SCOTT@ test1 >commit; Commit complete. 15:43:36 SCOTT@ test1 >select * from emp3_part; EMPNO SAL COMM SAL_PACK ---------- ---------- ---------- ---------- 10 1500 500 2000 20 3000 500 3500 30 4000 500 4500 40 6000 500 6500 15:43:44 SCOTT@ test1 >select * from emp3_part partition(sal_2000); no rows selected 15:44:01 SCOTT@ test1 >select * from emp3_part partition(sal_4000); EMPNO SAL COMM SAL_PACK ---------- ---------- ---------- ---------- 10 1500 500 2000 20 3000 500 3500 --通過以上對虛擬列的特性可以看出,Oracle采用虛擬列是占用了CPU計算時間,而節(jié)約了磁盤的存儲空間。
免責聲明:本站發(fā)布的內(nèi)容(圖片、視頻和文字)以原創(chuàng)、轉(zhuǎn)載和分享為主,文章觀點不代表本網(wǎng)站立場,如果涉及侵權(quán)請聯(lián)系站長郵箱:is@yisu.com進行舉報,并提供相關(guān)證據(jù),一經(jīng)查實,將立刻刪除涉嫌侵權(quán)內(nèi)容。