实体框架查询性能与原始 SQL 执行不同

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问题描述

我有一个关于实体框架查询执行性能的问题.

架构:

我有一个这样的表结构:

创建表 [dbo].[DataLogger]([ID] [bigint] IDENTITY(1,1) 非空，[项目ID] [bigint] NULL，约束 [PrimaryKey1] 主键集群([ID] ASC))创建表 [dbo].[DCDistributionBox]([ID] [bigint] IDENTITY(1,1) 非空，[DataLoggerID] [bigint] 非空，约束 [PrimaryKey2] 主键集群([ID] ASC))更改表 [dbo].[DCDistributionBox]添加约束 [FK_DCDistributionBox_DataLogger]外键([DataLoggerID]) 参考 [dbo].[DataLogger] ([ID])创建表 [dbo].[DCString]([ID] [bigint] IDENTITY(1,1) 非空，[DCDistributionBoxID] [bigint] 非空，[CurrentMPP] [十进制](18, 2) NULL,约束 [PrimaryKey3] 主键集群([ID] ASC))更改表 [dbo].[DCString]添加约束 [FK_DCString_DCDistributionBox]外键([DCDistributionBoxID]) 参考 [dbo].[DCDistributionBox] ([ID])创建表 [dbo].[StringData]([DCStringID] [bigint] 非空，[时间戳] [日期时间] 非空，[DCCurrent] [十进制](18, 2) NULL,约束 [PrimaryKey4] 主键集群([TimeStamp] DESC，[DCStringID] ASC))创建非集群索引 [TimeStamp_DCCurrent-NonClusteredIndex]ON [dbo].[StringData] ([DCStringID] ASC, [TimeStamp] ASC)包括([直流电流])

外键上的标准索引也存在(出于篇幅原因，我不想全部列出).

[StringData] 表具有以下存储统计信息:

数据空间:26,901.86 MB
行数:131,827,749
分区:真
分区数:62

用法:

我现在想对 [StringData] 表中的数据进行分组并进行一些聚合.

我创建了一个实体框架查询(查询的详细信息可以在

更新6(与@usr的回答有关):

我通过 SQL Server Profiler 创建了两个 showplan XML.

快速运行 (SSMS).SQLPlan

慢速运行 (EF).SQLPlan

Update 7(与@VladimirBaranov 的评论有关):

我现在运行一些与您的评论相关的测试用例.

首先，我通过使用一个新的计算列和一个匹配的 INDEXER 来消除订单操作的时间.这减少了与 DATEADD(MINUTE, DATEDIFF(MINUTE, 0, [TimeStamp] )/15* 15, 0) 相关的性能延迟.这里的详细信息.

结果如下:

纯EntityFramework查询:

for (int i = 0; i <3; i++){日期时间开始 = DateTime.UtcNow；var 结果 = model.StringDatas.AsNoTracking().Where(p => p.DCString.DCDistributionBox.DataLogger.ProjectID == projectID && p.TimeStamp15Minutes >= fromDate && p.TimeStamp15Minutes < tillDate).Select(d => 新{时间戳 = d.TimeStamp15 分钟，DCCurrentMpp = d.DCCurrent/d.DCString.CurrentMPP}).GroupBy(d => d.TimeStamp).Select(d => 新{时间戳 = d.Key，DCCurrentMppMin = d.Min(v => v.DCCurrentMpp),DCCurrentMppMax = d.Max(v => v.DCCurrentMpp),DCCurrentMppAvg = d.Average(v => v.DCCurrentMpp),DCCurrentMppStDev = DbFunctions.StandardDeviationP(d.Select(v => v.DCCurrentMpp))}).ToList();TimeSpan excecutionTimeSpan = DateTime.UtcNow - 开始；Debug.WriteLine("{0}th run pure EF: {1}", i, excecutionTimeSpan.ToString());}

第 0 次运行纯 EF:00:00:12.6460624

第 1 次运行纯 EF:00:00:11.0258393

第 2 次运行纯 EF:00:00:08.4171044

我现在使用 EF 生成的 SQL 作为 SQL 查询:

for (int i = 0; i <3; i++){日期时间开始 = DateTime.UtcNow；int 结果 = model.Database.ExecuteSqlCommand(@"SELECT1个AS [C1]，[GroupBy1].[K1] AS [TimeStamp15Minutes],[GroupBy1].[A1] AS [C2],[GroupBy1].[A2] AS [C3],[GroupBy1].[A3] AS [C4],[GroupBy1].[A4] 作为 [C5]从(选择[Project1].[TimeStamp15Minutes] AS [K1],MIN([Project1].[C1]) 作为 [A1],MAX([项目 1].[C1]) 作为 [A2],AVG([Project1].[C1]) AS [A3],STDEVP([项目 1].[C1]) 作为 [A4]从(选择[Extent1].[TimeStamp15Minutes] AS [TimeStamp15Minutes],[Extent1].[DCCurrent]/[Extent2].[CurrentMPP] AS [C1]FROM [dbo].[StringData] AS [Extent1]内部连接 [dbo].[DCString] AS [Extent2] ON [Extent1].[DCStringID] = [Extent2].[ID]INNER JOIN [dbo].[DCDistributionBox] AS [Extent3] ON [Extent2].[DCDistributionBoxID] = [Extent3].[ID]内部连接 [dbo].[DataLogger] AS [Extent4] ON [Extent3].[DataLoggerID] = [Extent4].[ID]WHERE ([Extent4].[ProjectID] = @p__linq__0) AND ([Extent1].[TimeStamp15Minutes] >= @p__linq__1) AND ([Extent1].[TimeStamp15Minutes] <@p__linq__2)) 作为 [项目1]GROUP BY [Project1].[TimeStamp15Minutes]) AS [GroupBy1];",新的 SqlParameter("p__linq__0", 20827),新的 SqlParameter("p__linq__1", fromDate),新的 SqlParameter("p__linq__2", tillDate));TimeSpan excecutionTimeSpan = DateTime.UtcNow - 开始；Debug.WriteLine("{0}th run: {1}", i, executionTimeSpan.ToString());}

第 0 次运行:00:00:00.8381200

第一次运行:00:00:00.6920736

第二次运行:00:00:00.7081006

和 OPTION(RECOMPILE):

for (int i = 0; i <3; i++){日期时间开始 = DateTime.UtcNow；int 结果 = model.Database.ExecuteSqlCommand(@"SELECT1个AS [C1]，[GroupBy1].[K1] AS [TimeStamp15Minutes],[GroupBy1].[A1] AS [C2],[GroupBy1].[A2] AS [C3],[GroupBy1].[A3] AS [C4],[GroupBy1].[A4] 作为 [C5]从(选择[Project1].[TimeStamp15Minutes] AS [K1],MIN([Project1].[C1]) 作为 [A1],MAX([项目 1].[C1]) 作为 [A2],AVG([Project1].[C1]) AS [A3],STDEVP([项目 1].[C1]) 作为 [A4]从(选择[Extent1].[TimeStamp15Minutes] AS [TimeStamp15Minutes],[Extent1].[DCCurrent]/[Extent2].[CurrentMPP] AS [C1]FROM [dbo].[StringData] AS [Extent1]内部连接 [dbo].[DCString] AS [Extent2] ON [Extent1].[DCStringID] = [Extent2].[ID]INNER JOIN [dbo].[DCDistributionBox] AS [Extent3] ON [Extent2].[DCDistributionBoxID] = [Extent3].[ID]内部连接 [dbo].[DataLogger] AS [Extent4] ON [Extent3].[DataLoggerID] = [Extent4].[ID]WHERE ([Extent4].[ProjectID] = @p__linq__0) AND ([Extent1].[TimeStamp15Minutes] >= @p__linq__1) AND ([Extent1].[TimeStamp15Minutes] <@p__linq__2)) 作为 [项目1]GROUP BY [Project1].[TimeStamp15Minutes]) AS [GroupBy1]选项(重新编译)；",新的 SqlParameter("p__linq__0", 20827),新的 SqlParameter("p__linq__1", fromDate),新的 SqlParameter("p__linq__2", tillDate));TimeSpan excecutionTimeSpan = DateTime.UtcNow - 开始；Debug.WriteLine("{0}th run: {1}", i, executionTimeSpan.ToString());}

第 0 次使用 RECOMPILE 运行:00:00:00.8260932

第一次运行 RECOMPILE:00:00:00.9139730

使用 RECOMPILE 进行第二次运行:00:00:01.0680665

在 SSMS 中执行相同的 SQL 查询(没有重新编译):

00:00:01.105

在 SSMS 中执行相同的 SQL 查询(使用 RECOMPILE):

00:00:00.902

我希望这是您需要的所有值.

解决方案

在这个答案中，我将重点放在最初的观察上:EF 生成的查询很慢，但是在 SSMS 中运行相同的查询时它很快.

对此行为的一种可能解释是参数嗅探.

<块引用>

SQL Server 在执行时使用了一个称为参数嗅探的过程有参数的存储过程.当...的时候过程被编译或重新编译，传入的值参数被评估并用于创建执行计划.那然后将值与执行计划一起存储在计划缓存中.在随后的执行，使用相同的值和相同的计划.

因此，EF 会生成一个参数很少的查询.第一次运行此查询时，服务器会使用第一次运行时生效的参数值为此查询创建一个执行计划.这个计划通常很不错.但是，稍后您使用其他参数值运行相同的 EF 查询.对于新的参数值，以前生成的计划可能不是最优的，查询会变慢.服务端继续使用之前的计划，因为还是同一个查询，只是参数值不同.

如果此时您获取查询文本并尝试直接在 SSMS 中运行它，服务器将创建一个新的执行计划，因为从技术上讲，它与 EF 应用程序发出的查询不同.即使一个字符差异就足够了，会话设置的任何更改也足以让服务器将查询视为新查询.因此，服务器在其缓存中针对看似相同的查询有两个计划.第一个慢"计划对于新的参数值很慢，因为它最初是为不同的参数值构建的.第二个快"计划是针对当前参数值构建的，所以速度很快.

Erland Sommarskog 的文章应用程序慢，SSMS 快更详细地解释了这个和其他相关领域.

有几种方法可以丢弃缓存的计划并强制服务器重新生成它们.更改表或更改表索引应该这样做 - 它应该丢弃与该表相关的所有计划，包括慢"和快".然后，您在 EF 应用程序中使用新的参数值运行查询并获得新的快速"计划.您在 SSMS 中运行查询并获得具有新参数值的第二个快速"计划.服务器仍然会生成两个计划，但现在两个计划都很快.

另一个变体是将 OPTION(RECOMPILE) 添加到查询中.使用此选项，服务器不会将生成的计划存储在其缓存中.因此，每次查询运行时，服务器都会使用实际参数值来生成(它认为)对于给定参数值最佳的计划.缺点是计划生成的额外开销.

请注意，例如，由于统计数据过时，服务器仍然可以使用此选项选择坏"计划.但是，至少，参数嗅探不会成为问题.

<小时>

想知道如何在 EF 生成的查询中添加 OPTION (RECOMPILE) 提示的人可以看看这个答案:

https://stackoverflow.com/a/26762756/4116017

I have a question about Entity Framework query execution performance.

Schema:

I have a table structure like this:

CREATE TABLE [dbo].[DataLogger]
(
    [ID] [bigint] IDENTITY(1,1) NOT NULL,
    [ProjectID] [bigint] NULL,
    CONSTRAINT [PrimaryKey1] PRIMARY KEY CLUSTERED ( [ID] ASC )
)

CREATE TABLE [dbo].[DCDistributionBox]
(
    [ID] [bigint] IDENTITY(1,1) NOT NULL,
    [DataLoggerID] [bigint] NOT NULL,
    CONSTRAINT [PrimaryKey2] PRIMARY KEY CLUSTERED ( [ID] ASC )
)

ALTER TABLE [dbo].[DCDistributionBox]
    ADD CONSTRAINT [FK_DCDistributionBox_DataLogger] 
    FOREIGN KEY([DataLoggerID]) REFERENCES [dbo].[DataLogger] ([ID])

CREATE TABLE [dbo].[DCString] 
(
    [ID] [bigint] IDENTITY(1,1) NOT NULL,
    [DCDistributionBoxID] [bigint] NOT NULL,
    [CurrentMPP] [decimal](18, 2) NULL,
    CONSTRAINT [PrimaryKey3] PRIMARY KEY CLUSTERED ( [ID] ASC )
)

ALTER TABLE [dbo].[DCString]
    ADD CONSTRAINT [FK_DCString_DCDistributionBox] 
    FOREIGN KEY([DCDistributionBoxID]) REFERENCES [dbo].[DCDistributionBox] ([ID])

CREATE TABLE [dbo].[StringData]
(
    [DCStringID] [bigint] NOT NULL,
    [TimeStamp] [datetime] NOT NULL,
    [DCCurrent] [decimal](18, 2) NULL,
    CONSTRAINT [PrimaryKey4] PRIMARY KEY CLUSTERED ( [TimeStamp] DESC, [DCStringID] ASC)
)

CREATE NONCLUSTERED INDEX [TimeStamp_DCCurrent-NonClusteredIndex] 
ON [dbo].[StringData] ([DCStringID] ASC, [TimeStamp] ASC)
INCLUDE ([DCCurrent])

Standard indexes on the foreign keys also exist (I don't want to list them all for space reasons).

The [StringData] table as has following storage stats:

Data space: 26,901.86 MB
Row count: 131,827,749
Partitioned: true
Partition count: 62

Usage:

I now want to group the data in the [StringData] table and do some aggregation.

I created an Entity Framework query (detailed infos to the query can be found here):

var compareData = model.StringDatas
    .AsNoTracking()
    .Where(p => p.DCString.DCDistributionBox.DataLogger.ProjectID == projectID && p.TimeStamp >= fromDate && p.TimeStamp < tillDate)
    .Select(d => new
    {
        TimeStamp = d.TimeStamp,
        DCCurrentMpp = d.DCCurrent / d.DCString.CurrentMPP
    })
    .GroupBy(d => DbFunctions.AddMinutes(DateTime.MinValue, DbFunctions.DiffMinutes(DateTime.MinValue, d.TimeStamp) / minuteInterval * minuteInterval))
    .Select(d => new
    {
        TimeStamp = d.Key,
        DCCurrentMppMin = d.Min(v => v.DCCurrentMpp),
        DCCurrentMppMax = d.Max(v => v.DCCurrentMpp),
        DCCurrentMppAvg = d.Average(v => v.DCCurrentMpp),
        DCCurrentMppStDev = DbFunctions.StandardDeviationP(d.Select(v => v.DCCurrentMpp))
    })
    .ToList();

The excecution timespan is exceptional long!?

Execution result: 92rows
Execution time: ~16000ms

Attempts:

I now took a look into the Entity Framework generated SQL query and looks like this:

DECLARE @p__linq__4 DATETIME = 0;
DECLARE @p__linq__3 DATETIME = 0;
DECLARE @p__linq__5 INT = 15;
DECLARE @p__linq__6 INT = 15;
DECLARE @p__linq__0 BIGINT = 20827;
DECLARE @p__linq__1 DATETIME = '06.02.2016 00:00:00';
DECLARE @p__linq__2 DATETIME = '07.02.2016 00:00:00';

SELECT 
1 AS [C1], 
[GroupBy1].[K1] AS [C2], 
[GroupBy1].[A1] AS [C3], 
[GroupBy1].[A2] AS [C4], 
[GroupBy1].[A3] AS [C5], 
[GroupBy1].[A4] AS [C6]
FROM ( SELECT 
    [Project1].[K1] AS [K1], 
    MIN([Project1].[A1]) AS [A1], 
    MAX([Project1].[A2]) AS [A2], 
    AVG([Project1].[A3]) AS [A3], 
    STDEVP([Project1].[A4]) AS [A4]
    FROM ( SELECT 
        DATEADD (minute, ((DATEDIFF (minute, @p__linq__4, [Project1].[TimeStamp])) / @p__linq__5) * @p__linq__6, @p__linq__3) AS [K1], 
        [Project1].[C1] AS [A1], 
        [Project1].[C1] AS [A2], 
        [Project1].[C1] AS [A3], 
        [Project1].[C1] AS [A4]
        FROM ( SELECT 
            [Extent1].[TimeStamp] AS [TimeStamp], 
            [Extent1].[DCCurrent] / [Extent2].[CurrentMPP] AS [C1]
            FROM    [dbo].[StringData] AS [Extent1]
            INNER JOIN [dbo].[DCString] AS [Extent2] ON [Extent1].[DCStringID] = [Extent2].[ID]
            INNER JOIN [dbo].[DCDistributionBox] AS [Extent3] ON [Extent2].[DCDistributionBoxID] = [Extent3].[ID]
            INNER JOIN [dbo].[DataLogger] AS [Extent4] ON [Extent3].[DataLoggerID] = [Extent4].[ID]
            WHERE (([Extent4].[ProjectID] = @p__linq__0) OR (([Extent4].[ProjectID] IS NULL) AND (@p__linq__0 IS NULL))) AND ([Extent1].[TimeStamp] >= @p__linq__1) AND ([Extent1].[TimeStamp] < @p__linq__2)
        )  AS [Project1]
    )  AS [Project1]
    GROUP BY [K1]
)  AS [GroupBy1]

I copied this SQL query into SSMS on the same machine, connected with same connection string as the Entity Framework.

The result is a very much improved performance:

Execution result: 92rows
Execution time: 517ms

I also do some loop runing test and the result is strange. The test looks like this

for (int i = 0; i < 50; i++)
{
    DateTime begin = DateTime.UtcNow;

    [...query...]

    TimeSpan excecutionTimeSpan = DateTime.UtcNow - begin;
    Debug.WriteLine("{0}th run: {1}", i, excecutionTimeSpan.ToString());
}

The result is very different and looks random(?):

0th run: 00:00:11.0618580
1th run: 00:00:11.3339467
2th run: 00:00:10.0000676
3th run: 00:00:10.1508140
4th run: 00:00:09.2041939
5th run: 00:00:07.6710321
6th run: 00:00:10.3386312
7th run: 00:00:17.3422765
8th run: 00:00:13.8620557
9th run: 00:00:14.9041528
10th run: 00:00:12.7772906
11th run: 00:00:17.0170235
12th run: 00:00:14.7773750

Question:

Why is Entity Framework query execution so slow? The resulting row count is really low and the raw SQL query shows a very fast performance.

Update 1:

I take care that its not a MetaContext or Model creation delay. Some other queries are executed on the same Model instance right before with good performance.

Update 2 (related to the answer of @x0007me):

Thanks for the hint but this can be eliminated by changing the model settings like this:

modelContext.Configuration.UseDatabaseNullSemantics = true;

The EF generated SQL is now:

SELECT 
1 AS [C1], 
[GroupBy1].[K1] AS [C2], 
[GroupBy1].[A1] AS [C3], 
[GroupBy1].[A2] AS [C4], 
[GroupBy1].[A3] AS [C5], 
[GroupBy1].[A4] AS [C6]
FROM ( SELECT 
    [Project1].[K1] AS [K1], 
    MIN([Project1].[A1]) AS [A1], 
    MAX([Project1].[A2]) AS [A2], 
    AVG([Project1].[A3]) AS [A3], 
    STDEVP([Project1].[A4]) AS [A4]
    FROM ( SELECT 
        DATEADD (minute, ((DATEDIFF (minute, @p__linq__4, [Project1].[TimeStamp])) / @p__linq__5) * @p__linq__6, @p__linq__3) AS [K1], 
        [Project1].[C1] AS [A1], 
        [Project1].[C1] AS [A2], 
        [Project1].[C1] AS [A3], 
        [Project1].[C1] AS [A4]
        FROM ( SELECT 
            [Extent1].[TimeStamp] AS [TimeStamp], 
            [Extent1].[DCCurrent] / [Extent2].[CurrentMPP] AS [C1]
            FROM    [dbo].[StringData] AS [Extent1]
            INNER JOIN [dbo].[DCString] AS [Extent2] ON [Extent1].[DCStringID] = [Extent2].[ID]
            INNER JOIN [dbo].[DCDistributionBox] AS [Extent3] ON [Extent2].[DCDistributionBoxID] = [Extent3].[ID]
            INNER JOIN [dbo].[DataLogger] AS [Extent4] ON [Extent3].[DataLoggerID] = [Extent4].[ID]
            WHERE ([Extent4].[ProjectID] = @p__linq__0) AND ([Extent1].[TimeStamp] >= @p__linq__1) AND ([Extent1].[TimeStamp] < @p__linq__2)
        )  AS [Project1]
    )  AS [Project1]
    GROUP BY [K1]
)  AS [GroupBy1]

So you can see the problem you described is now solved, but the execution time does not change.

Also, as you can see in the schema and the raw execution time, I used optimized structure with high optimized indexer.

Update 3 (related to the answer of @Vladimir Baranov):

I don't see why this can be related to query plan caching. Because in the MSDN is clearly descripted that the EF6 make use of query plan caching.

A simple test proof that the huge excecution time differenz is not related to the query plan caching (phseudo code):

using(var modelContext = new ModelContext())
{
    modelContext.Query(); //1th run activates caching

    modelContext.Query(); //2th used cached plan
}

As the result, both queries run with the same excecution time.

Update 4 (related to the answer of @bubi):

I tried to run the query that is generated by the EF as you descripted it:

int result = model.Database.ExecuteSqlCommand(@"SELECT 
    1 AS [C1], 
    [GroupBy1].[K1] AS [C2], 
    [GroupBy1].[A1] AS [C3], 
    [GroupBy1].[A2] AS [C4], 
    [GroupBy1].[A3] AS [C5], 
    [GroupBy1].[A4] AS [C6]
    FROM ( SELECT 
        [Project1].[K1] AS [K1], 
        MIN([Project1].[A1]) AS [A1], 
        MAX([Project1].[A2]) AS [A2], 
        AVG([Project1].[A3]) AS [A3], 
        STDEVP([Project1].[A4]) AS [A4]
        FROM ( SELECT 
            DATEADD (minute, ((DATEDIFF (minute, 0, [Project1].[TimeStamp])) / @p__linq__5) * @p__linq__6, 0) AS [K1], 
            [Project1].[C1] AS [A1], 
            [Project1].[C1] AS [A2], 
            [Project1].[C1] AS [A3], 
            [Project1].[C1] AS [A4]
            FROM ( SELECT 
                [Extent1].[TimeStamp] AS [TimeStamp], 
                [Extent1].[DCCurrent] / [Extent2].[CurrentMPP] AS [C1]
                FROM    [dbo].[StringData] AS [Extent1]
                INNER JOIN [dbo].[DCString] AS [Extent2] ON [Extent1].[DCStringID] = [Extent2].[ID]
                INNER JOIN [dbo].[DCDistributionBox] AS [Extent3] ON [Extent2].[DCDistributionBoxID] = [Extent3].[ID]
                INNER JOIN [dbo].[DataLogger] AS [Extent4] ON [Extent3].[DataLoggerID] = [Extent4].[ID]
                WHERE ([Extent4].[ProjectID] = @p__linq__0) AND ([Extent1].[TimeStamp] >= @p__linq__1) AND ([Extent1].[TimeStamp] < @p__linq__2)
            )  AS [Project1]
        )  AS [Project1]
        GROUP BY [K1]
    )  AS [GroupBy1]",
    new SqlParameter("p__linq__0", 20827),
    new SqlParameter("p__linq__1", fromDate),
    new SqlParameter("p__linq__2", tillDate),
    new SqlParameter("p__linq__5", 15),
    new SqlParameter("p__linq__6", 15));

Execution result: 92
Execution time: ~16000ms

It took exact as long as the normal EF query!?

Update 5 (related to the answer of @vittore):

I create a traced call tree, maybe it helps:

Update 6 (related to the answer of @usr):

I created two showplan XML via SQL Server Profiler.

Fast run (SSMS).SQLPlan

Slow run (EF).SQLPlan

Update 7 (related to the comments of @VladimirBaranov):

I now run some more test case related to your comments.

First I eleminate time taking order operations by using a new computed column and a matching INDEXER. This reduce the perfomance lag related to DATEADD(MINUTE, DATEDIFF(MINUTE, 0, [TimeStamp] ) / 15* 15, 0). Detail for how and why you can find here.

The Result look s like this:

Pure EntityFramework query:

for (int i = 0; i < 3; i++)
{
    DateTime begin = DateTime.UtcNow;
    var result = model.StringDatas
        .AsNoTracking()
        .Where(p => p.DCString.DCDistributionBox.DataLogger.ProjectID == projectID && p.TimeStamp15Minutes >= fromDate && p.TimeStamp15Minutes < tillDate)
        .Select(d => new
        {
            TimeStamp = d.TimeStamp15Minutes,
            DCCurrentMpp = d.DCCurrent / d.DCString.CurrentMPP
        })
        .GroupBy(d => d.TimeStamp)
        .Select(d => new
        {
            TimeStamp = d.Key,
            DCCurrentMppMin = d.Min(v => v.DCCurrentMpp),
            DCCurrentMppMax = d.Max(v => v.DCCurrentMpp),
            DCCurrentMppAvg = d.Average(v => v.DCCurrentMpp),
            DCCurrentMppStDev = DbFunctions.StandardDeviationP(d.Select(v => v.DCCurrentMpp))
        })
        .ToList();

        TimeSpan excecutionTimeSpan = DateTime.UtcNow - begin;
        Debug.WriteLine("{0}th run pure EF: {1}", i, excecutionTimeSpan.ToString());
}

0th run pure EF: 00:00:12.6460624

1th run pure EF: 00:00:11.0258393

2th run pure EF: 00:00:08.4171044

I now used the EF generated SQL as a SQL query:

for (int i = 0; i < 3; i++)
{
    DateTime begin = DateTime.UtcNow;
    int result = model.Database.ExecuteSqlCommand(@"SELECT 
        1 AS [C1], 
        [GroupBy1].[K1] AS [TimeStamp15Minutes], 
        [GroupBy1].[A1] AS [C2], 
        [GroupBy1].[A2] AS [C3], 
        [GroupBy1].[A3] AS [C4], 
        [GroupBy1].[A4] AS [C5]
        FROM ( SELECT 
            [Project1].[TimeStamp15Minutes] AS [K1], 
            MIN([Project1].[C1]) AS [A1], 
            MAX([Project1].[C1]) AS [A2], 
            AVG([Project1].[C1]) AS [A3], 
            STDEVP([Project1].[C1]) AS [A4]
            FROM ( SELECT 
                [Extent1].[TimeStamp15Minutes] AS [TimeStamp15Minutes], 
                [Extent1].[DCCurrent] / [Extent2].[CurrentMPP] AS [C1]
                FROM    [dbo].[StringData] AS [Extent1]
                INNER JOIN [dbo].[DCString] AS [Extent2] ON [Extent1].[DCStringID] = [Extent2].[ID]
                INNER JOIN [dbo].[DCDistributionBox] AS [Extent3] ON [Extent2].[DCDistributionBoxID] = [Extent3].[ID]
                INNER JOIN [dbo].[DataLogger] AS [Extent4] ON [Extent3].[DataLoggerID] = [Extent4].[ID]
                WHERE ([Extent4].[ProjectID] = @p__linq__0) AND ([Extent1].[TimeStamp15Minutes] >= @p__linq__1) AND ([Extent1].[TimeStamp15Minutes] < @p__linq__2)
            )  AS [Project1]
            GROUP BY [Project1].[TimeStamp15Minutes]
        )  AS [GroupBy1];",
    new SqlParameter("p__linq__0", 20827),
    new SqlParameter("p__linq__1", fromDate),
    new SqlParameter("p__linq__2", tillDate));

    TimeSpan excecutionTimeSpan = DateTime.UtcNow - begin;
    Debug.WriteLine("{0}th run: {1}", i, excecutionTimeSpan.ToString());
}

0th run: 00:00:00.8381200

1th run: 00:00:00.6920736

2th run: 00:00:00.7081006

and with OPTION(RECOMPILE):

for (int i = 0; i < 3; i++)
{
    DateTime begin = DateTime.UtcNow;
    int result = model.Database.ExecuteSqlCommand(@"SELECT 
        1 AS [C1], 
        [GroupBy1].[K1] AS [TimeStamp15Minutes], 
        [GroupBy1].[A1] AS [C2], 
        [GroupBy1].[A2] AS [C3], 
        [GroupBy1].[A3] AS [C4], 
        [GroupBy1].[A4] AS [C5]
        FROM ( SELECT 
            [Project1].[TimeStamp15Minutes] AS [K1], 
            MIN([Project1].[C1]) AS [A1], 
            MAX([Project1].[C1]) AS [A2], 
            AVG([Project1].[C1]) AS [A3], 
            STDEVP([Project1].[C1]) AS [A4]
            FROM ( SELECT 
                [Extent1].[TimeStamp15Minutes] AS [TimeStamp15Minutes], 
                [Extent1].[DCCurrent] / [Extent2].[CurrentMPP] AS [C1]
                FROM    [dbo].[StringData] AS [Extent1]
                INNER JOIN [dbo].[DCString] AS [Extent2] ON [Extent1].[DCStringID] = [Extent2].[ID]
                INNER JOIN [dbo].[DCDistributionBox] AS [Extent3] ON [Extent2].[DCDistributionBoxID] = [Extent3].[ID]
                INNER JOIN [dbo].[DataLogger] AS [Extent4] ON [Extent3].[DataLoggerID] = [Extent4].[ID]
                WHERE ([Extent4].[ProjectID] = @p__linq__0) AND ([Extent1].[TimeStamp15Minutes] >= @p__linq__1) AND ([Extent1].[TimeStamp15Minutes] < @p__linq__2)
            )  AS [Project1]
            GROUP BY [Project1].[TimeStamp15Minutes]
        )  AS [GroupBy1]
        OPTION(RECOMPILE);",
    new SqlParameter("p__linq__0", 20827),
    new SqlParameter("p__linq__1", fromDate),
    new SqlParameter("p__linq__2", tillDate));

    TimeSpan excecutionTimeSpan = DateTime.UtcNow - begin;
    Debug.WriteLine("{0}th run: {1}", i, excecutionTimeSpan.ToString());
}

0th run with RECOMPILE: 00:00:00.8260932

1th run with RECOMPILE: 00:00:00.9139730

2th run with RECOMPILE: 00:00:01.0680665

Same SQL query excecuted in SSMS (without RECOMPILE):

00:00:01.105

Same SQL query excecuted in SSMS (with RECOMPILE):

00:00:00.902

I hope this are all values you needed.

解决方案

In this answer I'm focusing on the original observation: the query generated by EF is slow, but when the same query is run in SSMS it is fast.

One possible explanation of this behaviour is Parameter sniffing.

SQL Server uses a process called parameter sniffing when it executes stored procedures that have parameters. When the procedure is compiled or recompiled, the value passed into the parameter is evaluated and used to create an execution plan. That value is then stored with the execution plan in the plan cache. On subsequent executions, that same value – and same plan – is used.

So, EF generates a query that has few parameters. The first time you run this query the server creates an execution plan for this query using values of parameters that were in effect in the first run. That plan is usually pretty good. But, later on you run the same EF query using other values for parameters. It is possible that for new values of parameters the previously generated plan is not optimal and the query becomes slow. The server keeps using the previous plan, because it is still the same query, just values of parameters are different.

If at this moment you take the query text and try to run it directly in SSMS the server will create a new execution plan, because technically it is not the same query that is issued by EF application. Even one character difference is enough, any change in the session settings is also enough for the server to treat the query as a new one. As a result the server has two plans for the seemingly same query in its cache. The first "slow" plan is slow for the new values of parameters, because it was originally built for different parameter values. The second "fast" plan is built for the current parameter values, so it is fast.

The article Slow in the Application, Fast in SSMS by Erland Sommarskog explains this and other related areas in much more details.

There are several ways to discard cached plans and force the server to regenerate them. Changing the table or changing the table indexes should do it - it should discard all plans that are related to this table, both "slow" and "fast". Then you run the query in EF application with new values of parameters and get a new "fast" plan. You run the query in SSMS and get a second "fast" plan with new values of parameters. The server still generates two plans, but both plans are fast now.

Another variant is adding OPTION(RECOMPILE) to the query. With this option the server would not store the generated plan in its cache. So, every time the query runs the server would use actual parameter values to generate the plan that (it thinks) would be optimal for the given parameter values. The downside is an added overhead of the plan generation.

Mind you, the server still could choose a "bad" plan with this option due to outdated statistics, for example. But, at least, parameter sniffing would not be a problem.

Those who wonder how to add OPTION (RECOMPILE) hint to the query that is generated by EF have a look at this answer:

https://stackoverflow.com/a/26762756/4116017

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