SQL Server multi computed columns reference the same column

env:Windows Server 2016

SQL Server 2012 SP1

看到這個標題一定覺得這有什麼好講的，直接加計算行欄位不就可以了嗎?

我當時也是覺得如此，但是還真的不可以~~

情境:有一張表TESTTB01有一create的欄位，資料形態是datetimeoffset，時區有+08:00，也有+07:00。需要用日期來判斷資料是哪一天。但是在不同時區造成日期的差異。

因此想利用計算型欄位來找出資料是屬於哪一天，並優化查詢效能。

資料約5億筆

1.直接在資料表參考同一欄位，建立3個計算行欄位。

command:

ALTER TABLE [dbo].[TESTTB01] ADD CreateDate AS (CONVERT(date, Created)) PERSISTED;
GO

ALTER TABLE [dbo].[TESTTB01] ADD CreateUTC AS (Created AT TIME ZONE 'UTC') PERSISTED;
GO

ALTER TABLE [dbo].[TESTTB01] ADD CreateUTCDate AS (CONVERT(date, (Created AT TIME ZONE 'UTC'))) PERSISTED;
GO

值行超過4小時，錯誤

Msg 4936, Level 16, State 1, Line 4

Computed column 'CreateUTC' in table 'TESTTB01' cannot be persisted because the column is non-deterministic.

Msg 4936, Level 16, State 1, Line 6

Computed column 'CreateUTCDate' in table 'TESTTB01' cannot be persisted because the column is non-deterministic.

2.嘗試重新建立空測試表:TESTTB01_NEW

command:

CREATE TABLE [dbo].[TESTTB01_NEW](
[Id] [uniqueidentifier] NOT NULL,
[Status] [int] NOT NULL,
[Created] [datetimeoffset](7) NOT NULL,
 CONSTRAINT [PK_TESTTB01_NEW] PRIMARY KEY NONCLUSTERED 
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO

3.建立function並設定計算型欄位

command:

CREATE FUNCTION dbo.FnCreateUTC(@DATE datetimeoffset)  
RETURNS datetimeoffset
AS  
BEGIN  
     DECLARE @CreateUTC DATETIMEOFFSET;  
     SELECT  @CreateUTC= (Created AT TIME ZONE 'UTC') FROM dbo.TESTTB01_NEW
     RETURN(@CreateUTC);  
END;  
GO

ALTER TABLE [dbo].[TESTTB01_NEW] ADD CreateUTC AS dbo.FnCreateUTC(created) PERSISTED;
GO

錯誤訊息，不可以使用PERSISTED

Msg 4936, Level 16, State 1, Line 49

Computed column 'CreateUTC' in table 'TESTTB01_NEW' cannot be persisted because the column is non-deterministic

4.不使用PERSISTED

command:

ALTER TABLE [dbo].[TESTTB01_NEW] ADD CreateUTC AS dbo.FnCreateUTC(created) ;
GO

5.再補上兩個計算型欄位，一個使用function，一個不使用function

command:

ALTER TABLE [dbo].[TESTTB01_NEW] ADD CreateUTC AS dbo.FnCreateUTC(created);
GO

ALTER TABLE [dbo].[TESTTB01_NEW] ADD CreateUTCDate AS (CONVERT(date, (Created AT TIME ZONE 'UTC')));
GO

結論:每一個欄位只能被一個計算型欄位參考，但是不侷限被function使用

在步驟3是一個不好的寫法，這支function效能很差，function內容改成以下效能好很多

     DECLARE @CreateUTCDate DATETIMEOFFSET;  

     SET @CreateUTCDate= (CONVERT(date, (@DATE AT TIME ZONE 'UTC')))

SQL Server multi computed columns reference the same column

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