Table变量和临时表区别

区别一:


<PRE lang=sql>CREATE table #T (s varchar(128))


DECLARE @T table (s varchar(128))


INSERT into #T select 'old value #'


INSERT into @T select 'old value @'


BEGIN transaction


UPDATE #T set s='new value #'


UPDATE @T set s='new value @'


ROLLBACK transaction


SELECT * from #T


SELECT * from @T


s


---------------


old value #




s


---------------


new value @</PRE>
这行代码一目了然,临时表能起到事务回滚的作用,但是table变量不可以.作者解释是table变量不在事务作用范围之内.所以当table变量即使遇到回滚命令,但是也不会真的执行回滚.

区别二:
任何含有临时表的存储过程是不能被预编译的.这在一个很长的存储过程中,优势会更加凸显.

区别三:
table变量作为变量只能在同范围内存在,不可能跨范围.还有就是table变量在内置的存储过程中或者exec(string)语句中是不可见的还有就是不能被用于INSERT/EXEC statements.

下面的例子我来演示如何对比临时表和table变量在存储过程中预编译的区别.
创建一个测试表:


CREATE table NUM (n int primary key, s varchar(128))


GO


SET nocount on


DECLARE @n int


SET @n=1000000


WHILE @n>0 begin


INSERT into NUM


SELECT @n,'Value: '+convert(varchar,@n)


SET @n=@n-1


END


GO

然后创建存储过程:T1


CREATE procedure T1


@total int


AS


CREATE table #T (n int, s varchar(128))


INSERT into #T select n,s from NUM


WHERE n%100>0 and n<=@total


DECLARE @res varchar(128)


SELECT @res=max(s) from NUM


WHERE n<=@total and


NOT exists(select * from #T


WHERE #T.n=NUM.n)


GO
这个存储过程的参数@Total给定的越大,那么执行的时间越长.

为精确测量存储过程执行时间,我使用了以下代码:


DECLARE @t1 datetime, @n int


SET @t1=getdate()


SET @n=100 – (**)


WHILE @n>0 begin


EXEC T1 1000 – (*)


SET @n=@n-1 end


SELECT datediff(ms,@t1,getdate())


GO
(*) 是存储过程参数.

现在我们给这个存储过程来第一次提速:个它加个主键


CREATE procedure T2


@total int


AS


CREATE table #T (n int primary key, s varchar(128))


INSERT into #T select n,s from NUM


WHERE n%100>0 and n<=@total


DECLARE @res varchar(128)


SELECT @res=max(s) from NUM


WHERE n<=@total and


NOT exists(select * from #T


WHERE #T.n=NUM.n)


GO
在下面的对比中你可以发现已经大大的提速了.

然后再来一次提速:给它加个聚集索引


CREATE procedure T3


@total int


AS


CREATE table #T (n int, s varchar(128))


INSERT into #T select n,s from NUM


WHERE n%100>0 and n<=@total


CREATE clustered index Tind on #T (n)


DECLARE @res varchar(128)


SELECT @res=max(s) from NUM


WHERE n<=@total and


NOT exists(select * from #T


WHERE #T.n=NUM.n)


GO

很惊讶速度已经提高了很多!

那好了,咱们再来测试一下table变量的速度吧


CREARE procedure V1


@total int


AS


DECLARE @V table (n int, s varchar(128))


INSERT into @V select n,s from NUM


WHERE n%100>0 and n<=@total


DECLARE @res varchar(128)


SELECT @res=max(s) from NUM


WHERE n<=@total and


NOT exists(select * from @V V


WHERE V.n=NUM.n)


GO

然后再来创建一个有主键的:


CREATE procedure V2


@total int


AS


DECLARE @V table (n int primary key, s varchar(128))


INSERT into @V select n,s from NUM


WHERE n%100>0 and n<=@total


DECLARE @res varchar(128)


SELECT @res=max(s) from NUM


WHERE n<=@total and


NOT exists(select * from @V V


WHEREre V.n=NUM.n)


GO<

然后咱们来看看我测试的结果吧!

Table 1, using SQL Server 2000, time in ms

Records	T1	T2	T3	V1	V2
10	0.7	1	13.5	0.6	0.8
100	1.2	1.7	14.2	1.2	1.3
1000	7.1	5.5	27	7	5.3
10000	72	57	82	71	48
100000	883	480	580	840	510
1000000	45056	6090	15220	20240	12010

But the real shock is when you try the same on SQL Server 2005:

Table 2

N	T1	T2	T3	V1	V2
10	0.5	0.5	5.3	0.2	0.2
100	2	1.2	6.4	61.8	2.5
1000	9.3	8.5	13.5	168	140
10000	67.4	79.2	71.3	17133	13910
100000	700	794	659	Too long!	Too long!
1000000	10556	8673	6440	Too long!	Too long!

发现对比在某些情况下sql2000的速度要比2005的要快上很多!

结论:
没有通用的规则指导你什么时候用临时表什么时候用table变量.
将复杂逻辑的存储过程移植到sql2005的时候,要格外小心!他可能比2000的效率要慢上好几十倍的!
在你的实际测试中,请测试两个极端:销量数据和超大量数据.