使用分组的 Linq to SQL 性能

Question

我的问题是关于 Linq to SQL 性能，我有一个 SQL 字符串并将其转换为 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 SQL 分析器中的 LINQ 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 太慢，生成查询的执行计划与 SQL 查询相比，人工编写的 SQL 查询为 7%，而 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

我认为问题在于您在 OrderBy G.First 语句中访问每个组的行并在 Linq-to-SQL 中触发 N+1 行为，您可以尝试以下操作：

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 它解决了这个问题，我同意你的观点，这个问题与 G.First 有关。

我根据您的回答更改了我的 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，但查询速度要快得多，分析器给了我 55% 的书面查询和 45% 的新生成查询，比原始字符串查询更快。

非常感谢您的帮助。