LINQ 和实体框架 - 避免子查询

Question

我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 使用了多个内部子查询，这些子查询似乎在DB 中执行得非常糟糕，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，其查询性能非常糟糕，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生成查询的执行时间为~10 seconds。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

执行时间接近即时；下面30ms。

问题显然在于我使用 LINQ 编写好的 EF 查询的能力，但无论我尝试做什么（使用 Linqpad 进行测试）我都无法像我一样使用 LINQ\EF 编写类似的高性能查询可以手写。我尝试查询 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完成工作。

如何编写一个模仿上面显示的手写SQL 的查询？如何控制EF 生成的实际查询？最重要的是：我怎样才能让Linq2SQL 和Entity Framework 在我想使用Joins 而不是嵌套子查询时使用。

Answer 1

EF 从您提供的 LINQ 表达式生成 SQL，并且您不能期望这种转​​换会完全解开您放入表达式中的任何结构以优化它。在您的情况下，您创建了一个表达式树，每个项目都将使用导航属性来汇总与项目相关的一些子值。如您所见，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来做到这一点（这也是您在手工制作的 SQL 中所做的）：

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);

基本思想是在受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。

然后对连接的值 (x) 进行分组，并在每个组中执行 SubValueSum 的总和。

最后，ordering 和TOP(10) 被应用。

生成的 SQL 仍然包含子查询，但与您的查询生成的 SQL 相比，我希望它更有效：

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