T-SQL中SELECT与SET的速度

Question

我一直认为,对于T-SQL中的单变量赋值,这set是最好的方法,原因有两个:

• 它是变量赋值的ANSI标准
• 它实际上比执行SELECT(对于单个变量)更快

所以...

SELECT @thingy = 'turnip shaped'

变

SET @thingy  = 'turnip shaped'

但速度有多快,快吗？我真的会注意到差异吗？

Answer 1

单次运行时SET更快.你可以很容易地证明这一点.它是否有所作为取决于你,但我更喜欢SET,因为如果所有代码都在进行,我没有看到SELECT的重点.我更喜欢将SELECT限制在表,视图等的SELECT语句中.

这是一个示例脚本,其运行次数设置为1:

SET NOCOUNT ON

DECLARE @runs int
DECLARE @i int, @j int
SET @runs = 1
SET @i = 0
SET @j = 0

DECLARE @dtStartDate datetime, @dtEndDate datetime


WHILE @runs > 0
    BEGIN
        SET @j = 0
        SET @dtStartDate = CURRENT_TIMESTAMP
        WHILE @j < 1000000
            BEGIN
                SET @i = @j
                SET @j = @j + 1
            END
        SELECT @dtEndDate = CURRENT_TIMESTAMP
        SELECT DATEDIFF(millisecond, @dtStartDate, @dtEndDate) AS SET_MILLISECONDS


        SET @j = 0
        SET @dtStartDate = CURRENT_TIMESTAMP
        WHILE @j < 1000000
            BEGIN
                SELECT @i = @j
                SET @j = @j + 1
            END
        SELECT @dtEndDate = CURRENT_TIMESTAMP
        SELECT DATEDIFF(millisecond, @dtStartDate, @dtEndDate) AS SELECT_MILLISECONDS

        SET @runs = @runs - 1
    END

结果:

运行#1:

SET_MILLISECONDS

5093

SELECT_MILLISECONDS

5186

运行#2:

SET_MILLISECONDS

4876

SELECT_MILLISECONDS

5466

运行#3:

SET_MILLISECONDS

4936

SELECT_MILLISECONDS

5453

运行#4:

SET_MILLISECONDS

4920

SELECT_MILLISECONDS

5250

运行#5:

SET_MILLISECONDS

4860

SELECT_MILLISECONDS

5093

奇怪的是,如果你将运行次数设置为10,则SET开始落后.

这是一个10运行的结果:

SET_MILLISECONDS

5140

SELECT_MILLISECONDS

5266

SET_MILLISECONDS

5250

SELECT_MILLISECONDS

5466

SET_MILLISECONDS

5220

SELECT_MILLISECONDS

5280

SET_MILLISECONDS

5376

SELECT_MILLISECONDS

5280

SET_MILLISECONDS

5233

SELECT_MILLISECONDS

5453

SET_MILLISECONDS

5343

SELECT_MILLISECONDS

5423

SET_MILLISECONDS

5360

SELECT_MILLISECONDS

5156

SET_MILLISECONDS

5686

SELECT_MILLISECONDS

5233

SET_MILLISECONDS

5436

SELECT_MILLISECONDS

5500

SET_MILLISECONDS

5610

SELECT_MILLISECONDS

5266