为什么实体框架使用重复的 SQL 生成嵌套的 SELECT 查询？

Question

I'm using Entity Framework logging to look at the SQL generated by my queries. In many cases the SQL that's produced has SELECT statements operating on subqueries that are identical to the original query. For example:

The following EF code:

context.PodcastItems
.Where(p => p.ID == sourceFeedUpdateItemId)
.Select(p => new NewPodcastItemSubsetDTO
{
   ID = p.ID,
   ItemID = p.ItemID,
   ItemTitle = p.ItemTitle,
   ItemSummary = p.ItemSummary,
   PermalinkUrl = p.PermalinkUrl,
   ItemPublishedDate = p.ItemPublishedDate,
   AudioFileUrl = p.AudioFileUrl,
   FileSize = p.FileSize,
   Duration = p.Duration
}).SingleOrDefault();

generates the following SQL:

SELECT
    [Limit1].[ID] AS [ID],
    [Limit1].[ItemID] AS [ItemID],
    [Limit1].[ItemTitle] AS [ItemTitle],
    [Limit1].[ItemSummary] AS [ItemSummary],
    [Limit1].[PermalinkUrl] AS [PermalinkUrl],
    [Limit1].[ItemPublishedDate] AS [ItemPublishedDate],
    [Limit1].[AudioFileUrl] AS [AudioFileUrl],
    [Limit1].[FileSize] AS [FileSize],
    [Limit1].[Duration] AS [Duration]
    FROM ( SELECT TOP (2)
        [Extent1].[ID] AS [ID],
        [Extent1].[ItemID] AS [ItemID],
        [Extent1].[ItemTitle] AS [ItemTitle],
        [Extent1].[ItemSummary] AS [ItemSummary],
        [Extent1].[PermalinkUrl] AS [PermalinkUrl],
        [Extent1].[AudioFileUrl] AS [AudioFileUrl],
        [Extent1].[FileSize] AS [FileSize],
        [Extent1].[Duration] AS [Duration],
        [Extent1].[ItemPublishedDate] AS [ItemPublishedDate]
        FROM [dbo].[PodcastItems] AS [Extent1]
        WHERE [Extent1].[ID] = @p__linq__0
    )  AS [Limit1]

and this code:

string content = context.PodcastItems
.Where(p => p.ID == sourceFeedUpdateItemId)
.Select(p => p.ItemContentCleansed)
.SingleOrDefault();

produces:

SELECT
    [Limit1].[ItemContentCleansed] AS [ItemContentCleansed]
    FROM ( SELECT TOP (2)
        [Extent1].[ItemContentCleansed] AS [ItemContentCleansed]
        FROM [dbo].[PodcastItems] AS [Extent1]
        WHERE [Extent1].[ID] = @p__linq__0
    )  AS [Limit1]
SELECT
    [Limit1].[ItemContentCleansed] AS [ItemContentCleansed]
    FROM ( SELECT TOP (2)
        [Extent1].[ItemContentCleansed] AS [ItemContentCleansed]
        FROM [dbo].[PodcastItems] AS [Extent1]
        WHERE [Extent1].[ID] = @p__linq__0
    )  AS [Limit1]
SELECT
    [Limit1].[ItemContentCleansed] AS [ItemContentCleansed]
    FROM ( SELECT TOP (2)
        [Extent1].[ItemContentCleansed] AS [ItemContentCleansed]
        FROM [dbo].[PodcastItems] AS [Extent1]
        WHERE [Extent1].[ID] = @p__linq__0
    )  AS [Limit1]
SELECT
    [Limit1].[ItemContentCleansed] AS [ItemContentCleansed]
    FROM ( SELECT TOP (2)
        [Extent1].[ItemContentCleansed] AS [ItemContentCleansed]
        FROM [dbo].[PodcastItems] AS [Extent1]
        WHERE [Extent1].[ID] = @p__linq__0
    )  AS [Limit1]
SELECT
    [Limit1].[ItemContentCleansed] AS [ItemContentCleansed]
    FROM ( SELECT TOP (2)
        [Extent1].[ItemContentCleansed] AS [ItemContentCleansed]
        FROM [dbo].[PodcastItems] AS [Extent1]
        WHERE [Extent1].[ID] = @p__linq__0
    )  AS [Limit1]

Sure seems like a lot of extra cycles are spent to accomplish...nothing?

Why might this be happening?

Answer 1

Check EDMX file, maybe your entity is defined via DefiningQuery like this:

<!-- SSDL content -->
<edmx:StorageModels>
  <Schema Namespace="SchoolModel.Store" Alias="Self" Provider="System.Data.SqlClient" ProviderManifestToken="2008" xmlns:store="http://schemas.microsoft.com/ado/2007/12/edm/EntityStoreSchemaGenerator" xmlns="http://schemas.microsoft.com/ado/2009/11/edm/ssdl">
    <EntityContainer Name="SchoolModelStoreContainer">
       <EntitySet Name="GradeReport" EntityType="SchoolModel.Store.GradeReport">
          <DefiningQuery>
            SELECT CourseID, Grade, FirstName, LastName
            FROM StudentGrade
          </DefiningQuery>
      </EntitySet>

You need to replace it without DefiningQuery with EntitySet only:

 <EntitySet Name="Course" EntityType="SchoolModel.Store.Course" store:Type="Tables" Schema="dbo" />

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