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，我們會創建數據庫視圖，如果我們需要在這個視圖上使用索引，我們會創建基于函數的索引。

我們從Oracle官方文檔中，找到下面對于虛擬列技術的描述。

“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，不存儲在數據庫中，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

3. 可以在虛擬列上建索引，oracle的函數索引就類似。

4. 可以在虛擬列上建約束

案例：

1、創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建的，使用的關鍵字有 VIRTUAL(不過這個關鍵字是可選的)，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：

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據：

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據到虛擬列：

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

虛擬列的數據會自動計算生成

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，而節約了磁盤的存儲空間。

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