我必须创建一个连接3 IEnumerable(2个数据表,一个列表)的LINQ查询.我还必须“外部加入”我的2个加入.

我真的很喜欢第二个,因为它更易读并且可与SQL媲美.但是与第一个相比,我遇到了性能问题(长3倍),可读性较差,但速度更快.

有人可以向我解释如何优化第二个查询,以及为什么它们有很大不同,但是返回相同的数据(我对LINQ陌生,并且很难理解每个LINQ-TO-SQL / LAMBDA / SAMBA实用程序

旧的,真的很快,但是不太容易听

 var Detailled_payroll = 
        from MainData in dt_BasicPayrollData.AsEnumerable()
           //joining (Externalize later...) with a list of days
           join MyDays in gdt_Liste_JourFérié 
              on MainData.Field<DateTime>("DENTREE") equals MyDays 
                  into MyDays_join //why?
           //joining (Externalize later...) with a list (datatable) of things
           join MyFilter in gdt_MyFilter.AsEnumerable()
              on MainData.Field<string>("KEY") equals MyFilter.Field<string>("KEY") 
                  into MyFilter_join   //why?

           //I think we "outer join" here, forcing the existing of a missing join                             
           from MyDays in MyDays_join.DefaultIfEmpty()
           from MyFilter in MyFilter_join.DefaultIfEmpty()

           let tmp = MainData.Field<string>("CEmploye") //dont remember why..

           select new Blabla()
           { 
                 EmpNumber=MainData.Field<string>("NoEmploye"),
                 IsInSpecialDays = MyDays == Default(DateTime) ? "NO" : "YES",
                 KeyWasFound = MyFilter == null ? "NO" : "YES"
           }

这是“新”的一个：

    var Detailled_payroll = 
        from MainData in dt_BasicPayrollData.AsEnumerable()
        //joining (and externalize...) with a list (datatable) of things
        from MyFilter in MyGlobalVariable.gdt_FiltreEnCours.AsEnumerable()
           .Where(x => x.Field<string>("KEY") == MainData.Field<string>("KEY"))
           .DefaultIfEmpty()
        //joining (and externalize...) with a list (List<DateTime>) of days
        from MyDays in gdt_Liste_JourFérié  
           .Where(x => x == MainData.Field<DateTime>("DENTREE"))
           .DefaultIfEmpty()

        select new Blabla()
          { 
                 EmpNumber=MainData.Field<string>("NoEmploye"),
                 IsInSpecialDays = MyDays == Default(DateTime) ? "NO" : "YES",
                 KeyWasFound = MyFilter == null ? "NO" : "YES"
           }

>“旧”查询将在3秒钟内返回结果.
>“新”查询将在10秒内返回结果.

两者具有完全相同的结果数据.

有任何想法吗？

解决方法:

简而言之,在第一个中您具有联接,在第二个中您具有笛卡尔积,然后对其进行过滤.