我真的很难在我的应用程序中调整我的 Entity Framework 生成的查询之一。这是非常基本的查询，但出于某种原因，EF 使用多个内部子查询，这似乎在 DB 中执行得很糟糕，而不是使用连接。

这是我的 LINQ 代码:

Projects.Select(proj => new ProjectViewModel()
                {
                    Name = proj.Name,
                    Id = proj.Id,
                    Total = proj.Subvalue.Where(subv =>
                        subv.Created >= startDate
                        && subv.Created <= endDate
                        &&
                        (subv.StatusId == 1 ||
                         subv.StatusId == 2))
                        .Select(c => c.SubValueSum)
                        .DefaultIfEmpty()
                        .Sum()
                })
                .OrderByDescending(c => c.Total)
                .Take(10);

EF 生成具有多个子查询的非常复杂的查询，其查询性能很糟糕，如下所示:

SELECT TOP (10) 
[Project3].[Id] AS [Id], 
[Project3].[Name] AS [Name], 
[Project3].[C1] AS [C1]
FROM ( SELECT 
    [Project2].[Id] AS [Id], 
    [Project2].[Name] AS [Name], 
    [Project2].[C1] AS [C1]
    FROM ( SELECT 
        [Extent1].[Id] AS [Id], 
        [Extent1].[Name] AS [Name], 
        (SELECT 
            SUM([Join1].[A1]) AS [A1]
            FROM ( SELECT 
                CASE WHEN ([Project1].[C1] IS NULL) THEN cast(0 as decimal(18)) ELSE [Project1].[SubValueSum] END AS [A1]
                FROM   ( SELECT 1 AS X ) AS [SingleRowTable1]
                LEFT OUTER JOIN  (SELECT 
                    [Extent2].[SubValueSum] AS [SubValueSum], 
                    cast(1 as tinyint) AS [C1]
                    FROM [dbo].[Subvalue] AS [Extent2]
                    WHERE ([Extent1].[Id] = [Extent2].[Id]) AND ([Extent2].[Created] >= '2015-08-01') AND ([Extent2].[Created] <= '2015-10-01') AND ([Extent2].[StatusId] IN (1,2)) ) AS [Project1] ON 1 = 1
            )  AS [Join1]) AS [C1]
        FROM [dbo].[Project] AS [Extent1]
        WHERE ([Extent1].[ProjectCountryId] = 77) AND ([Extent1].[Active] = 1)
    )  AS [Project2]
)  AS [Project3]
ORDER BY [Project3].[C1] DESC;

EF 生成的查询的执行时间为~10 秒。但是当我像这样手动编写查询时:

select 
    TOP (10)
    Proj.Id,
    Proj.Name,
    SUM(Subv.SubValueSum) AS Total
from 
    SubValue as Subv
left join
    Project as Proj on Proj.Id = Subv.ProjectId
where
    Subv.Created > '2015-08-01' AND Subv.Created <= '2015-10-01' AND Subv.StatusId IN (1,2)
group by
    Proj.Id,
    Proj.Name
order by 
    Total DESC

执行时间接近瞬间；低于 30ms。

问题显然在于我使用 LINQ 编写良好的 EF 查询的能力，但无论我尝试做什么(使用 Linqpad 进行测试)我都做不到使用 LINQ\EF 编写类似的高性能查询，因为我可以手写。我已经尝试查询 SubValue 表和 Project 表，但结果基本相同:多个无效的嵌套子查询而不是单个连接来完成工作。

我如何编写一个查询来模仿上面显示的手写 SQL？如何控制 EF 生成的实际查询？最重要的是:当我想要代替嵌套子查询时，如何让 Linq2SQL 和 Entity Framework 使用 Joins。

最佳答案

EF 从您提供的 LINQ 表达式生成 SQL，您不能指望此转换会完全解开您为优化表达式而放入的任何内容的结构。在您的情况下，您已经为每个项目创建了一个表达式树，该表达式树将使用一个导航属性来对与该项目相关的一些子值求和。正如您所发现的那样，这会导致嵌套子查询。

要改进生成的 SQL，您需要避免在对子值执行所有操作之前从项目导航到子值，您可以通过创建连接(这也是您在手工制作的 SQL 中所做的)来做到这一点:

var query = from proj in context.Project
            join s in context.SubValue.Where(s => s.Created >= startDate && s.Created <= endDate && (s.StatusId == 1 || s.StatusId == 2)) on proj.Id equals s.ProjectId into s2
            from subv in s2.DefaultIfEmpty()
            select new { proj, subv } into x
            group x by new { x.proj.Id, x.proj.Name } into g
            select new {
              g.Key.Id,
              g.Key.Name,
              Total = g.Select(y => y.subv.SubValueSum).Sum()
            } into y
            orderby y.Total descending
            select y;
var result = query.Take(10);

基本思想是在 where 子句限制的子值上加入项目。要执行左连接，您需要 DefaultIfEmpty() 但您已经知道了。

然后将连接的值 (x) 分组，并在每个组中执行 SubValueSum 的求和。

最后，应用排序和 TOP(10)。

生成的 SQL 仍然包含子查询，但我希望它比您的查询生成的 SQL 更高效:

SELECT TOP (10)
    [Project1].[Id] AS [Id],
    [Project1].[Name] AS [Name],
    [Project1].[C1] AS [C1]
    FROM ( SELECT
        [GroupBy1].[A1] AS [C1],
        [GroupBy1].[K1] AS [Id],
        [GroupBy1].[K2] AS [Name]
        FROM ( SELECT
            [Extent1].[Id] AS [K1],
            [Extent1].[Name] AS [K2],
            SUM([Extent2].[SubValueSum]) AS [A1]
            FROM  [dbo].[Project] AS [Extent1]
            LEFT OUTER JOIN [dbo].[SubValue] AS [Extent2] ON ([Extent2].[Created] >= @p__linq__0) AND ([Extent2].[Created] <= @p__linq__1) AND ([Extent2].[StatusId] IN (1,2)) AND ([Extent1].[Id] = [Extent2].[ProjectId])
            GROUP BY [Extent1].[Id], [Extent1].[Name]
        )  AS [GroupBy1]
    )  AS [Project1]
    ORDER BY [Project1].[C1] DESC

关于c# - LINQ 和 Entity Framework - 避免子查询，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/32737701/