SQL Server：两个类似的DISTINCT查询导致不同的执行计划

Question

我的问题是关于SQL Server为DISTINCT查询构建的执行计划。

我的数据库中有一个名为MainTable的表，其中有两列：Id (PK, int, not null)和Name (nvarchar(200), not null)。

我运行以下查询（由实体框架生成）

SELECT 
    [Project2].[Id] AS [Id]
FROM   
    (SELECT 
         [Distinct1].[Id] AS [Id]
     FROM   
         (SELECT DISTINCT 
              [Extent3].[Id] AS [Id]
          FROM   
              [dbo].[TableA] AS [Extent1]
          INNER JOIN 
              [dbo].[TableB] AS [Extent2] ON [Extent1].[Id] = [Extent2].[TableAId]
          INNER JOIN 
              [dbo].[MainTable] AS [Extent3] ON [Extent2].[MainTableId] = [Extent3].[Id]
          WHERE  
              [Extent1].[TableCId] = 48 /*  @p__linq__0 */) AS [Distinct1]) AS [Project2]

这会产生效率非常低的execution plan。

它使用嵌套循环并添加TOP语句，尽管它在原始查询中不存在。在表格中的2-3k记录上运行需要几秒钟，并进行大量读取。

我尝试在DISTINCT列上使用Name运行类似的查询，结果似乎要好得多。所以这个查询：

SELECT [Project2].[Name]          AS [Name]
FROM   (SELECT [Distinct1].[Name]          AS [Name]
    FROM   (SELECT DISTINCT [Extent3].[Name]          AS [Name]
            FROM   [dbo].[TableA] AS [Extent1]
                   INNER JOIN [dbo].[TableB] AS [Extent2]
                     ON [Extent1].[Id] = [Extent2].[TableAId]
                   INNER JOIN [dbo].[MainTable] AS [Extent3]
                     ON [Extent2].[MainTableId] = [Extent3].[Id]
            WHERE  [Extent1].[TableCId] = 48 /* @p__linq__0 */) AS [Distinct1]) AS [Project2]

在我的案例中效率更高

excecution plan

使用哈希匹配

所以我想知道在DISTINCT和Id上制作Name之间的区别是什么，为什么要添加TOP语句以及如何使第一个查询更有效率？

由于

更新

使用SELECT DISTINCT [Extent2].[MainTableId]代替SELECT DISTINCT [Extent3].[Id]会产生第二个更有效的执行计划。