Question

如何让Linq-to-Entities提供商真正执行GROUP BY？无论我做什么，它总是生成比实际GROUP BY慢得多的SQL。例如：

var foo = (from x in Context.AccountQuantities
  where x.AccountID == 27777
  group x by x.StartDate
  into groups
  select new
    {
      groups.FirstOrDefault().StartDate,
      Quantity = groups.Max(y => y.Quantity)
    }).ToList();

转换为：

SELECT 
1 AS [C1], 
[Project4].[C1] AS [C2], 
[Project4].[C2] AS [C3]
FROM ( SELECT 
    [Project3].[C1] AS [C1], 
    (SELECT 
        MAX([Extent3].[Quantity]) AS [A1]
        FROM [dbo].[AccountQuantities] AS [Extent3]
        WHERE (27777 = [Extent3].[AccountID]) AND ([Project3].[StartDate] = [Extent3].[StartDate])) AS [C2]
    FROM ( SELECT 
        [Distinct1].[StartDate] AS [StartDate], 
        (SELECT TOP (1) 
            [Extent2].[StartDate] AS [StartDate]
            FROM [dbo].[AccountQuantities] AS [Extent2]
            WHERE (27777 = [Extent2].[AccountID]) AND ([Distinct1].[StartDate] = [Extent2].[StartDate])) AS [C1]
        FROM ( SELECT DISTINCT 
            [Extent1].[StartDate] AS [StartDate]
            FROM [dbo].[AccountQuantities] AS [Extent1]
            WHERE 27777 = [Extent1].[AccountID]
        )  AS [Distinct1]
    )  AS [Project3]
)  AS [Project4]

我怎样才能让它执行此操作呢？

SELECT
AccountQuantities.StartDate,
MAX(AccountQuantities.Quantity)
FROM AccountQuantities
WHERE AccountID=27777
GROUP BY StartDate

再次注意，在执行的内容中缺少任何GROUP BY。我很好地利用EF非理想地优化了一些东西，但是这个数字要慢一些，我找不到任何方法来说服它真正做GROUP BY，这对我们来说是个主要问题！

Answer 1

使用groups.Key而不是groups.FirstOrDefault（）。StartDate

var foo = (from x in Context.AccountQuantities
           where x.AccountID == 27777
           group x by x.StartDate
           into groups
           select new
           {
             groups.Key,
             Quantity = groups.Max(y => y.Quantity)
            }).ToList();

L2E：GroupBy总是被转换为Distincts，而不是Group By

1 个答案: