提高 LINQ To Entities 查询（聚合）的性能

Question

我有这个 EF 模型：

class Reception
{
     public string Code { get; set; }
     public virtual List<Row> { get; set; }
}

class Row 
{
     public string Item { get; set; }
     public int Quantity { get; set; }
     public float Weight { get; set; }
}

有没有办法改进以下 LINQ To Entities 查询？

dbContext.Receptions.Select(r => new 
{
     code = r.Code,
     quantitySum = r.Rows.Sum(e => e.Quantity),
     weightSum = r.Rows.Sum(e => e.Weight),
});

我担心做两次“r.Rows”部分。

我不应该担心吗？

Answer 1

如果基于 GroupBy，聚合查询（尤其是包含多个聚合的查询）可以更好地转换为 SQL，因为这是聚合的自然 SQL 构造。

因此，如果您想要更好的翻译并忍受代码可读性，可以将有问题的查询转换为left outer join + group by，如下所示：

var query = dbContext.Receptions
    .SelectMany(r => r.Rows.DefaultIfEmpty(), (r, e) => new
    {
        r.Code,
        Quantity = (int?)e.Quantity ?? 0,
        Weight = (float?)e.Weight ?? 0,
    })
    .GroupBy(e => e.Code, (key, g) => new
    {
        code = key,
        quantitySum = g.Sum(e => e.Quantity),
        weightSum = g.Sum(e => e.Weight),
    });

翻译成这样的东西

SELECT 
    1 AS [C1], 
    [GroupBy1].[K1] AS [Code], 
    [GroupBy1].[A1] AS [C2], 
     CAST( [GroupBy1].[A2] AS real) AS [C3]
    FROM ( SELECT 
        [Join1].[K1] AS [K1], 
        SUM([Join1].[A1]) AS [A1], 
        SUM([Join1].[A2]) AS [A2]
        FROM ( SELECT 
            [Extent1].[Code] AS [K1], 
            CASE WHEN ([Extent2].[Quantity] IS NULL) THEN 0 ELSE [Extent2].[Quantity] END AS [A1], 
            CASE WHEN ([Extent2].[Weight] IS NULL) THEN cast(0 as real) ELSE [Extent2].[Weight] END AS [A2]
            FROM  [dbo].[Receptions] AS [Extent1]
            LEFT OUTER JOIN [dbo].[Rows] AS [Extent2] ON [Extent1].[Code] = [Extent2].[Reception_Code]
        )  AS [Join1]
        GROUP BY [K1]
    )  AS [GroupBy1]

对于此特定查询，您可以从 EF6 获得最好的结果。

Answer 2

通常，EF 应该将其转换为相当有效的数据库查询，同时在数据库端使用 SUM 函数。但如果您想确定，请使用您的 SQL Server Profiler 来分析查询。

我为您快速运行了这段代码，这就是 EF 所做的：

SELECT 
    [Project2].[Id] AS [Id], 
    [Project2].[Code] AS [Code], 
    [Project2].[C1] AS [C1], 
     CAST( [Project2].[C2] AS real) AS [C2]
FROM 
    (SELECT 
        [Project1].[Id] AS [Id], 
        [Project1].[Code] AS [Code], 
        [Project1].[C1] AS [C1], 
        (SELECT SUM([Extent3].[Weight]) AS [A1] FROM [dbo].[Rows] AS [Extent3] WHERE [Project1].[Id] = [Extent3].[Reception_Id]) AS [C2]
    FROM 
        (SELECT 
            [Extent1].[Id] AS [Id], 
            [Extent1].[Code] AS [Code], 
            (SELECT SUM([Extent2].[Quantity]) AS [A1] FROM [dbo].[Rows] AS [Extent2] WHERE [Extent1].[Id] = [Extent2].[Reception_Id]) AS [C1]
        FROM 
            [dbo].[Receptions] AS [Extent1]
        )AS [Project1]
    )AS [Project2]

当然，我们可以编写一个更好的查询，使用更少的子选择，但实际上，这个查询并不是 SQL Server 会遇到的。