LINQ到与分组

Question

我的问题是关于LINQ to SQL的性能SQL性能，我有一个SQL字符串，并将其转换成的LINQ to SQL：

SQL查询：

SELECT CONVERT(VARCHAR(10), ClockIn, 103) AS ClockDate, MIN(ClockIn) AS ClockIn, MAX(ClockOut) AS ClockOut, SUM(DATEDIFF(MINUTE, ClockIn, ClockOut)) AS [TotalTime] 
FROM TimeLog 
WHERE (EmployeeId = 10) 
GROUP BY CONVERT(VARCHAR(10), ClockIn, 103) 
ORDER BY ClockIn DESC 

LINQ查询：

From u In objDC.TimeLogs 
Where u.EmployeeId = 10 
Group By Key = New With {u.ClockIn.Year, u.ClockIn.Month, u.ClockIn.Day} Into G = Group 
Order By G.First.ClockIn Descending 
Select New With {.ClockDate = Key.Day & "/" & Key.Month & "/" & Key.Year, 
.ClockIn = G.Min(Function(p) p.ClockIn), 
.ClockOut = G.Max(Function(p) p.ClockOut), 
.TotalTime = G.Sum(Function(p) SqlMethods.DateDiffMinute(p.ClockIn, p.ClockOut))} 

从LINQ在SQL事件探查器生成的查询字符串是：

SELECT [t4].[value] AS [ClockDate], [t4].[value2] AS [ClockIn2], [t4].[value22] AS [ClockOut], [t4].[value3] AS [TotalTime] 
FROM (
SELECT ((((CONVERT(NVarChar,[t3].[value32])) + '/') + (CONVERT(NVarChar,[t3].[value222]))) + '/') + (CONVERT(NVarChar,[t3].[value22])) AS [value], [t3].[value] AS [value2], [t3].[value2] AS [value22], [t3].[value3], [t3].[value22] AS [value222], [t3].[value222] AS [value2222], [t3].[value32] 
FROM (
SELECT MIN([t2].[ClockIn]) AS [value], MAX([t2].[ClockOut]) AS [value2], SUM([t2].[value]) AS [value3], [t2].[value2] AS [value22], [t2].[value22] AS [value222], [t2].[value3] AS [value32] 
FROM (
SELECT DATEDIFF(Minute, [t1].[ClockIn], [t1].[ClockOut]) AS [value], [t1].[EmployeeId], [t1].[value] AS [value2], [t1].[value2] AS [value22], [t1].[value3], [t1].[ClockIn], [t1].[ClockOut] 
FROM (
SELECT DATEPART(Year, [t0].[ClockIn]) AS [value], DATEPART(Month, [t0].[ClockIn]) AS [value2], DATEPART(Day, [t0].[ClockIn]) AS [value3], [t0].[ClockIn], [t0].[ClockOut], [t0].[EmployeeId] 
FROM [dbo].[TimeLog] AS [t0] 
) AS [t1] 
) AS [t2] 
WHERE [t2].[EmployeeId] = 10 
GROUP BY [t2].[value2], [t2].[value22], [t2].[value3] 
) AS [t3] 
) AS [t4] 
ORDER BY (
SELECT [t6].[ClockIn] 
FROM (
SELECT TOP (1) [t5].[ClockIn] 
FROM [dbo].[TimeLog] AS [t5] 
WHERE ((([t4].[value222] IS NULL) AND (DATEPART(Year, [t5].[ClockIn]) IS NULL)) OR (([t4].[value222] IS NOT NULL) AND (DATEPART(Year, [t5].[ClockIn]) IS NOT NULL) AND ((([t4].[value222] IS NULL) AND (DATEPART(Year, [t5].[ClockIn]) IS NULL)) OR (([t4].[value222] IS NOT NULL) AND (DATEPART(Year, [t5].[ClockIn]) IS NOT NULL) AND ([t4].[value222] = DATEPART(Year, [t5].[ClockIn])))))) AND ((([t4].[value2222] IS NULL) AND (DATEPART(Month, [t5].[ClockIn]) IS NULL)) OR (([t4].[value2222] IS NOT NULL) AND (DATEPART(Month, [t5].[ClockIn]) IS NOT NULL) AND ((([t4].[value2222] IS NULL) AND (DATEPART(Month, [t5].[ClockIn]) IS NULL)) OR (([t4].[value2222] IS NOT NULL) AND (DATEPART(Month, [t5].[ClockIn]) IS NOT NULL) AND ([t4].[value2222] = DATEPART(Month, [t5].[ClockIn])))))) AND ((([t4].[value32] IS NULL) AND (DATEPART(Day, [t5].[ClockIn]) IS NULL)) OR (([t4].[value32] IS NOT NULL) AND (DATEPART(Day, [t5].[ClockIn]) IS NOT NULL) AND ((([t4].[value32] IS NULL) AND (DATEPART(Day, [t5].[ClockIn]) IS NULL)) OR (([t4]. 
[value32] IS NOT NULL) AND (DATEPART(Day, [t5].[ClockIn]) IS NOT NULL) AND ([t4].[value32] = DATEPART(Day, [t5].[ClockIn])))))) AND ([t5].[EmployeeId] = 10) 
) AS [t6] 
) DESC 

LINQ to SQL过于缓慢，生成的查询与SQL查询相比的执行计划对于人工编写的SQL查询为7％，对于Linq生成的查询为97％。

我的Linq to SQL查询出了什么问题？还是Linq的性能和限制？

Answer 1

我认为这个问题是您访问的每个组的列在你的OrderBy G.First声明，并触发LINQ到SQL中的N + 1的行为，你可以尝试这样的：

var query = objDC.TimeLogs 
      .Where(c => c.EmployeeId == 10) 
      .GroupBy(c => c.ClockIn.Date) 
      .OrderBy(g => g.Key) 
      .Select(g => new 
      { 
       Date = g.Key, 
       ClockIn = g.Min(c => c.ClockIn), 
       ClockOut = g.Max(c => c.ClockOut), 
      }) 
      .Select(g => new 
      { 
       g.Date, 
       g.ClockIn, 
       g.ClockOut, 
       TotalTime = g.ClockOut - g.ClockIn 
      }); 

Answer 2

同样，这是基于Guillaume推荐的linq查询。

非常感谢Guillaume它解决了问题，我同意你的问题与G.First有关。

我根据你的回答改变了我的LINQ查询：

From u In objDC.TimeLogs 
Where u.EmployeeId = 10 
Group By key = New With {u.ClockIn.Date} Into G = Group 
Order By key.Date Descending 
Select New With { 
    .ClockDate = key.Date, 
    .ClockIn = G.Min(Function(p) p.ClockIn), 
    .ClockOut = G.Max(Function(p) p.ClockOut), 
    .TotalTime = G.Sum(Function(p) SqlMethods.DateDiffMinute(p.ClockIn, p.ClockOut))/60} 

我得到了相同的结果，但查询是速度更快，并剖析给了我书面查询55％和45％为新生成的查询，它从原始字符串查询更快。

非常感谢您的帮助。