Cont()与Where().Count()有时性能差别如此之大！

复制代码 代码如下:

SELECT COUNT(*) AS [value]
FROM (
SELECT
(CASE
WHEN ([t1].[CompanyID] = ([t0].[ID])) AND ([t1].[IsDeleted] = @p0) AND (([t1].[AuditStatus] = @p1) OR ([t1].[AuditStatus] = @p2)) THEN 1
WHEN NOT (([t1].[CompanyID] = ([t0].[ID])) AND ([t1].[IsDeleted] = @p0) AND (([t1].[AuditStatus] = @p1) OR ([t1].[AuditStatus] = @p2))) THEN 0
ELSE NULL
END) AS [value]
FROM [Company_Product_Maintain] AS [t1]
) AS [t2]
WHERE [t2].[value] = 1

复制代码 代码如下:

SELECT COUNT(*) AS [value]
FROM [GasSNS_Company_Equipment_Maintain] AS [t1]
WHERE ([t1].[CompanyID] = ([t0].[ID])) AND ([t1].[IsDeleted] = @p0) AND (([t1].[AuditStatus] = @p1) OR ([t1].[AuditStatus] = @p2))

 图1为Count结果，用了35秒，哇塞！

图2为Where(条件).Count()结果，同样的数据只用了4秒钟，差了10倍！

然后为了取值方面我还是加入三元运算，ContentStatus=Product_Maintain.Where(C => C.CompanyID == company.ID && C.IsDeleted == 0 && (C.AuditStatus == 0 || C.AuditStatus == 4)).Count()>0？"产品有更新":""。结果如下：

真的是Count()与 Where()区别，不可能这么大差距吧？于是我单写

 Product_Maintain.Where(C => C.IsDeleted == 0 && (C.AuditStatus == 0 || C.AuditStatus == 4)).Count()  与

 Product_Maintain.Count(C => C.IsDeleted == 0 && (C.AuditStatus == 0 || C.AuditStatus == 4))   

发现速度差不多，生成的代码是一样的。

复制代码 代码如下:

SELECT COUNT(*) AS [value]
FROM [GasSNS_Company_Equipment_Maintain] AS [t0]
WHERE ([t0].[IsDeleted] = @p0) AND (([t0].[AuditStatus] = @p1) OR ([t0].[AuditStatus] = @p2))

复制代码 代码如下:

//效率低版本：
from company in Company
select new
{
contacter = v.ContacterID,
count = Product_Maintain.Count(C => C.CompanyID == company.ID &&C.IsDeleted == 0 && (C.AuditStatus == 0 || C.AuditStatus == 4))
}

复制代码 代码如下:

//效率高版本：
from company in Company
select new
{
contacter = v.ContacterID,
count = Product_Maintain.Where(C =>C.CompanyID == company.ID && C.IsDeleted == 0 && (C.AuditStatus == 0 || C.AuditStatus == 4)).Count()
}

      总结到此，望各位看官以后要注意！本人入园两年来，第一发在首页，请各位看官不吝赐教！

      谢谢各位看官的指点，声明下以上查询图都LinqPad查询结果截图。至于为啥4秒左右为LinqPad查询时间，Linq生成Sql语句在Sql Server中执行不到1秒，以下截图作解释：