2

我有一个查询：

var contactInfos = from person in persons

                    join tempDesiredCar in desiredCars on person.contact_id equals tempDesiredCar.groupEntity_id
                                   into tempDesiredCars
                    from desiredCar in tempDesiredCars.DefaultIfEmpty()
                    select new {name = person.name, car = desiredCar.name};

此代码转换为 SQL：

SELECT [t1].[name], [t19].[name] AS [car]
FROM [dbo].[Person] AS [t1]
CROSS APPLY ((
        SELECT NULL AS [EMPTY]
        ) AS [t6]
    OUTER APPLY (
        SELECT [t18].[name]
        FROM (
            SELECT [t17].[contact_id], [t17].[name]
            FROM (
                SELECT [t7].[contact_id], [t11].[name]
                FROM [dbo].[DesiredCar] AS [t7]
                INNER JOIN (
                    SELECT MAX([t9].[value]) AS [value]
                    FROM (
                        SELECT [t8].[id] AS [value], [t8].[contact_id]
                        FROM [dbo].[DesiredCar] AS [t8]
                        ) AS [t9]
                    GROUP BY [t9].[contact_id]
                    ) AS [t10] ON ([t7].[id]) = [t10].[value]
                LEFT OUTER JOIN [dbo].[FamilyModel] AS [t11] ON [t11].[id] = [t7].[model]
                WHERE [t7].[model] IS NOT NULL
                UNION
                SELECT [t12].[contact_id], [t16].[name]
                FROM [dbo].[DesiredCar] AS [t12]
                INNER JOIN (
                    SELECT MAX([t14].[value]) AS [value]
                    FROM (
                        SELECT [t13].[id] AS [value], [t13].[contact_id]
                        FROM [dbo].[DesiredCar] AS [t13]
                        ) AS [t14]
                    GROUP BY [t14].[contact_id]
                    ) AS [t15] ON ([t12].[id]) = [t15].[value]
                LEFT OUTER JOIN [dbo].[CarBrand] AS [t16] ON [t16].[id] = [t12].[carBrand_id]
                WHERE [t12].[carBrand_id] IS NOT NULL
                ) AS [t17]
            ) AS [t18]
        where [t1].[contact_id] = [t18].[contact_id]
        ) AS [t19])

该代码使用 Apply。如果将 Apply 替换为 Left Join代码会在几次内运行得更快。如何强制 Linq 使用左连接生成代码？

4

3 回答 3

1

我找到了 LINQ 使用应用运算符的原因。操作数正在检查该连接中是否存在与外键的依赖关系。如果它只是一个表 - linq 使用“左连接”。如果子查询包含 Group By、Order By 以及在“equals”中使用的列，则 LINQ 使用“Outer apply”。在 LINQ JOIN 中使用简单的子查询。

于 2013-01-11T09:07:42.233 回答
0

我找不到为什么 a) join 和 apply 产生不同的计划和 b) LINQ 完全使用 apply 的原因。这是一个奇怪的案例。

我尝试提出解决方法，希望它们有效：

//project only the needed columns
var contactInfos = from person in persons
                    join tempDesiredCar in
                         desiredCars.Select(x => new { x.groupEntity_id, x.name })
                         on person.contact_id equals tempDesiredCar.groupEntity_id
                         into tempDesiredCars
                    from desiredCar in tempDesiredCars.DefaultIfEmpty()
                    select new { name = person.name, car = desiredCar.name };

//change join to from
var contactInfos = from person in persons
                   from desiredCar in
                         desiredCars.Select(x => new { x.groupEntity_id, x.name })
                                    .Where(x => person.contact_id = x.groupEntity_id)
                                    .DefaultIfEmpty()
                    select new { name = person.name, car = desiredCar.name };

这些只是推测性的重写。我希望他们能巧合地工作......我知道这不是解决问题的好方法，但在这种奇怪的情况下，这是我能提供的最好的方法。

于 2012-12-28T15:47:19.417 回答
0

我检查了查询计划。

应用计划使用嵌套循环将表连接在一起。对于 Person 表中的每条记录，它会读取 DesiredCars 表中的每条记录。你真的需要在数据库中找到每个人想要的汽车吗？如果不是，请考虑先过滤一下 Person 表。

连接计划使用 Hashmatching 将表连接在一起。它读取两个表一次，生成一个表的映射，并在第二个表上使用该映射。优化器指出了一些索引建议。这些建议可能对这两个计划都有帮助。

        <MissingIndexes>
          <MissingIndexGroup Impact="12.6549">
            <MissingIndex Database="[AudiCRM]" Schema="[dbo]" Table="[DesiredCar]">
              <ColumnGroup Usage="INEQUALITY">
                <Column Name="[carBrand_id]" ColumnId="13" />
              </ColumnGroup>
              <ColumnGroup Usage="INCLUDE">
                <Column Name="[id]" ColumnId="1" />
                <Column Name="[contact_id]" ColumnId="2" />
              </ColumnGroup>
            </MissingIndex>
          </MissingIndexGroup>
          <MissingIndexGroup Impact="12.6549">
            <MissingIndex Database="[AudiCRM]" Schema="[dbo]" Table="[DesiredCar]">
              <ColumnGroup Usage="INEQUALITY">
                <Column Name="[model]" ColumnId="3" />
              </ColumnGroup>
              <ColumnGroup Usage="INCLUDE">
                <Column Name="[id]" ColumnId="1" />
                <Column Name="[contact_id]" ColumnId="2" />
              </ColumnGroup>
            </MissingIndex>
          </MissingIndexGroup>
        </MissingIndexes>
于 2012-12-30T04:30:56.380 回答