Linq 实体框架似乎不必要地连接同一个表两次

更新这可能已经修复：

针对我的实体的相当简单的LINQ语句会导致不必要的复杂SQL。稍后将介绍更多信息，以下是设置：

表格

出版

• PublicationId（主键）
• TopicId（fk到主题表）
• ReceiptCount（针对查询性能进行非规范化）
• 插入日期

收据

• 接收方ID（pk）
• PublicationId（fk到上表）
• 插入日期

LINQ

var query = from r in context.Receipts.Include("Publication")
            where r.DateInserted < lagDate
            && r.ReceiptId > request.AfterReceiptId
            && r.Publication.TopicId == topicEntity.TopicId
            && r.Publication.ReceiptCount > 1
            select r;

var query=context.Receipts.Include（“发布”）中的r
其中r.DateInsertedrequest.AfterReceiptId
&&r.Publication.TopicId==topicEntity.TopicId
&&r.Publication.ReceiptCount>1
选择r；

SQL

exec sp_executesql N'SELECT TOP (25) 
[Project1].[ReceiptId] AS [ReceiptId], 
[Project1].[PublicationId] AS [PublicationId], 
[Project1].[DateInserted] AS [DateInserted], 
[Project1].[DateReceived] AS [DateReceived], 
[Project1].[PublicationId1] AS [PublicationId1], 
[Project1].[PayloadId] AS [PayloadId], 
[Project1].[TopicId] AS [TopicId], 
[Project1].[BrokerType] AS [BrokerType], 
[Project1].[DateInserted1] AS [DateInserted1], 
[Project1].[DateProcessed] AS [DateProcessed], 
[Project1].[DateUpdated] AS [DateUpdated], 
[Project1].[PublicationGuid] AS [PublicationGuid], 
[Project1].[ReceiptCount] AS [ReceiptCount]
FROM ( SELECT 
    [Extent1].[ReceiptId] AS [ReceiptId], 
    [Extent1].[PublicationId] AS [PublicationId], 
    [Extent1].[DateInserted] AS [DateInserted], 
    [Extent1].[DateReceived] AS [DateReceived], 
    [Extent3].[PublicationId] AS [PublicationId1], 
    [Extent3].[PayloadId] AS [PayloadId], 
    [Extent3].[TopicId] AS [TopicId], 
    [Extent3].[BrokerType] AS [BrokerType], 
    [Extent3].[DateInserted] AS [DateInserted1], 
    [Extent3].[DateProcessed] AS [DateProcessed], 
    [Extent3].[DateUpdated] AS [DateUpdated], 
    [Extent3].[PublicationGuid] AS [PublicationGuid], 
    [Extent3].[ReceiptCount] AS [ReceiptCount]
    FROM   [dbo].[Receipt] AS [Extent1]
    INNER JOIN [dbo].[Publication] AS [Extent2] ON [Extent1].[PublicationId] = [Extent2].[PublicationId]
    LEFT OUTER JOIN [dbo].[Publication] AS [Extent3] ON [Extent1].[PublicationId] = [Extent3].[PublicationId]
    WHERE ([Extent2].[ReceiptCount] > 1) AND ([Extent1].[DateInserted] < @p__linq__0) AND ([Extent1].[ReceiptId] > @p__linq__1) AND ([Extent2].[TopicId] = @p__linq__2)
)  AS [Project1]
ORDER BY [Project1].[ReceiptId] ASC',N'@p__linq__0 datetime,@p__linq__1 int,@p__linq__2 int',@p__linq__0='2012-09-05 19:39:21:510',@p__linq__1=4458824,@p__linq__2=90

exec sp_executesql N'选择顶部（25）
[Project1]。[ReceiptId]作为[ReceiptId]，
[Project1]。[PublicationId]作为[PublicationId]，
[Project1]。[DateInserted]作为[DateInserted]，
[Project1][DateReceived]作为[DateReceived]，
[Project1][PublicationId1]作为[PublicationId1]，
[Project1]。[PayloadId]作为[PayloadId]，
[Project1]。[TopicId]作为[TopicId]，
[Project1]。[BrokerType]作为[BrokerType]，
[Project1]。[DateInserted1]作为[DateInserted1]，
[Project1]。[DateProcessed]作为[DateProcessed]，
[Project1]。[DateUpdated]为[DateUpdated]，
[Project1]。[PublicationGuid]作为[PublicationGuid]，
[Project1][ReceiptCount]作为[ReceiptCount]
从（选择
[Extent1]。[ReceiptId]作为[ReceiptId]，
[Extent1]。[PublicationId]作为[PublicationId]，
[Extent1]。[DateInserted]作为[DateInserted]，
[Extent1]。[DateReceived]作为[DateReceived]，
[Extent3]。[PublicationId]作为[PublicationId1]，
[Extent3]。[PayloadId]作为[PayloadId]，
[Extent3]。[TopicId]作为[TopicId]，
[Extent3]。[BrokerType]作为[BrokerType]，
[Extent3]。[DateInserted]作为[DateInserted1]，
[Extent3]。[DateProcessed]作为[DateProcessed]，
[Extent3]。[DateUpdate]作为[DateUpdate]，
[Extent3]。[PublicationGuid]作为[PublicationGuid]，
[Extent3][ReceiptCount]作为[ReceiptCount]
从[dbo].[Receipt]作为[Extent1]
内部联接[dbo].[Publication]作为[Extent1].[PublicationId]=[Extent2].[PublicationId]上的[Extent2]
[Extent1].[PublicationId]=[Extent3].[PublicationId]上的[Extent3]左外部联接[dbo].[Publication]为[Extent3]
其中（[Extent2].[ReceiptCount]>1）和（[Extent1].[DateInserted]<@p_uulinq_u0）和（[Extent1].[ReceiptId]>@p_ulinq_u1）和（[Extent2].[TopicId]=@p_ulinq_u2）
)AS[Project1]
由[Project1].[ReceiptId]ASC，N'@p_uLinq_u0 datetime，@p_uLinq_u1 int，@p_uLinq_u2 int'，@p_uLinq_u0='2012-09-05 19:39:21:510'，@p_uLinq_u1=4458824，@p_uLinq_u2=90订购

问题

发布被加入两次：

由于

.Include（“Publication”）

由于

的where

而导致内部联接

如果我完全从SQL中删除[Extent2]，并更改使用[Extent3]的位置位，我会得到相同的结果。因为我没有在我的实体上使用延迟加载，所以我必须

.Include（“Publication”）

。。。有什么解决办法吗

---

我使用的是EF4，但从NuGet抓取了EF5，看看它是否被修复，但它产生了相同的结果（尽管我不知道如何判断我的EDMX是否真的在使用EF5）。

不过，还有一个解决办法。它可能不是最优雅的解决方案，但它确实满足了您的需求；它只生成一个连接

更改：

var query = from r in context.Receipts.Include("Publication")    
            where r.DateInserted < lagDate 
            && r.ReceiptId > request.AfterReceiptId 
            && r.Publication.TopicId == topicEntity.TopicId 
            && r.Publication.ReceiptCount > 1 
            select r; 

---

我将通过如下更改代码来优化前一个人的答案，这样就不需要连接，这样您就不必知道需要连接哪些列，也不必在连接条件更改时更改LINQ。这本应该是不必要的，但微软目前并不专注于修复其sql代码生成

    var query = from pub in context.Publications
                from r in pub.Reciepts
                where r.DateInserted < lagDate 
                && r.ReceiptId > request.AfterReceiptId 
                && pub.TopicId == topicEntity.TopicId 
                && pub.ReceiptCount > 1 
                select new {
                       Receipt = r,
                       Publication = pub
                };

var query=来自context.Publications中的发布
从酒吧里的r收到
其中r.DateInsertedrequest.AfterReceiptId
&&pub.TopicId==topicEntity.TopicId
&&pub.ReceiptCount>1
选择新的{
收据=r，
发布=发布
};

使用临时变量（例如，

let

pub=r.Publication）可以避免这种行为

var query=context.Receipts中的r
让pub=r.Publication//使用临时变量
其中r.DateInsertedrequest.AfterReceiptId
&&pub.TopicId==topicEntity.TopicId
&&pub.ReceiptCount>1
选择新建{r，pub}；

我认为where和include都在进行连接，并且没有简化为左连接。您应该将此作为EF Codeplex项目的改进发布。然而，我认为这将是相当低的优先级，因为我认为SQL对于这种“甚至”的NHibernate来说是相当有效的。将其视为自动查询生成的特殊性。数据库引擎可能会对其进行优化。“不过观察得不错。@GertArnold它实际上导致了性能问题。”。SQL事件探查器报告的持续时间为700-900毫秒，如果我删除它，我会得到0-6毫秒。男孩！坏消息。看起来它还是需要改进的。@Langdon你要删除哪一个子句？我认为这将被解释为一个

内部连接

，而不是

左连接

@edze它应该是一个内部连接

a内部连接b作为b1左连接b作为b2

（提示此问题的双连接）将永远不会为

b2

返回空行，因为所有缺少

b

行的

a

都已被第一个连接过滤掉。也就是说，同样的想法应该可以在中实现

foreach ( var item in query)
{
    //see that item.Publication is not null
    if(item.Receipt != null && item.Receipt.Publication != null)
    {
        //do work based on a valid Publication
    }
    else
    {
        //do work based on no linked Publication
    }
}

    var query = from pub in context.Publications
                from r in pub.Reciepts
                where r.DateInserted < lagDate 
                && r.ReceiptId > request.AfterReceiptId 
                && pub.TopicId == topicEntity.TopicId 
                && pub.ReceiptCount > 1 
                select new {
                       Receipt = r,
                       Publication = pub
                };

var query = from r in context.Receipts
            let pub = r.Publication // using a temp variable
            where r.DateInserted < lagDate
            && r.ReceiptId > request.AfterReceiptId
            && pub.TopicId == topicEntity.TopicId
            && pub.ReceiptCount > 1
            select new { r, pub };