我发现“count( ) over()”会比“select count( ) from table”快得多。
例如
使用 count( *) 结束
with CTE as(
select col_A,col_B,totalNumber=count(*) over() from table1 where conditions..)
select totalNumber from CTE
使用 select count( *) from ( 或者也使用 count(1) )
select count(*) from table1 where conditions..
在我对SQL Server 2K5的本地测试中，如果搜索条件复杂且返回的行很大，则count( ) over* 将快 4 倍。
但是为什么 count(**) over 执行得这么快呢？
谢谢你的建议。
万斯
更新
我认为真的错过了一些细节：
实际上，我使用“准备语句” sql 进行测试，例如：
exec sp_executesql N'SELECT count(*)
FROM tableA WHERE (aaa in(@P0))
AND (bbb like @P1)',
N'@P0 nvarchar(4000),@P1 nvarchar(4000)',N'XXXXXXX-XXXX-XXX',N'%AAA%'
Execution Plan says "HashMatch" cost 61%, others is "index seek". And the execution time will be 1484ms and logical reads around 4000.
与
SELECT count(*)
FROM tableA WHERE (aaa in('XXXXXXX-XXXX-XXX'))
AND (bbb like '%AAA%')
Execution plan says "clustered index seek" cost 98%. And the execution time is 46ms and logical reads will be 8000.
如果将第一个 sql 更改为：
exec sp_executesql N'with CTE as(
SELECT total=count(*) over ()
FROM tableA WHERE (aaa in(@P0))
AND (bbb like @P1)) select top 1total from cte',
N'@P0 nvarchar(4000),@P1 nvarchar(4000)',N'XXXXXXX-XXXX-XXX',N'%AAA%'
Execution plan says "clustered index seek 58%', no "hashmatch join" occurs.
And the execution time is 15ms and logical reads is: 8404.
那么，“哈希匹配连接”是否对性能有很大的开销？