为什么Entity Framework 6为简单查找生成复杂的SQL查询？ [英] Why does Entity Framework 6 generate complex SQL queries for simple lookups?

问题描述

我有这个LINQ查询

dbContext.Customers.Where(c => c.AssetTag == assetTag).Count();

或

(from c in dbContext.Customers
 where c.AssetTag == assetTag
 select c).Count();

生成的SQL是

SELECT 
[GroupBy1].[A1] AS [C1]
FROM ( SELECT 
    COUNT(1) AS [A1]
    FROM [dbo].[Customer] AS [Extent1]
    WHERE (([Extent1].[AssetTag] = @p__linq__0) AND ( NOT ([Extent1].[AssetTag] IS NULL    OR @p__linq__0 IS NULL))) OR (([Extent1].[AssetTag] IS NULL) AND (@p__linq__0 IS NULL))
)  AS [GroupBy1]

那么为什么LINQ为简单的where语句生成这样复杂的SQL？

So why does LINQ generate such complex SQL for a simple where statement?

推荐答案

在EF6中，数据库为null语义是默认的比较语义。请注意，这是EF5中默认设置的更改。在EF5中，这个标志被埋在 ObjectContext.ContextOptions.UseCSharpNullComparisonBehavior ，默认情况下，EF将使用Linq对象比较语义。在EF6中，它在DbContext上显示为 DbContext.Configuration.UseDatabaseNullSemantics 。你可以找到更多的细节 here

In EF6 the database null semantics is the default comparison semantics. Note that this is a change to the default setting in EF5. In EF5 this flag was buried in ObjectContext.ContextOptions.UseCSharpNullComparisonBehavior and by default EF would use Linq to Object comparison semantics. In EF6 it is exposed on DbContext as DbContext.Configuration.UseDatabaseNullSemantics. You can find more details here

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