优化LINQ To Entities查询

Question

好吧，假设我有两个实体A和B.对于每个用户，可能有0个或更多个A，具有UserId（外键），GroupName和Name属性的唯一组合。实体A还具有“价值”属性。同样对于每个用户，可能有0个或更多个B，具有用户ID（再次，外键）和“发生”属性。

我的任务是查找所有发生属性超过7天以前的B或具有发生的属性比现在更多 - 特定A属性中的小时数。此代码似乎完美地工作：

DateTime now = DateTime.UtcNow; 
DateTime expired = now - TimeSpan.FromDays(7d); 

using (DatabaseContext context = DatabaseContext.Create()) 
{ 
    IQueryable<A> aQ = context.As.Where(a => a.GroupName == "Notifications" && s.Name == "Retention"); 

    IQueryable<B> bQ = context.Bs.Where(
    n => aQ.Any(a => a.UserId == b.UserId) ? 
     b.Occurred < EntityFunctions.AddHours(now, -aQ.FirstOrDefault(a => a.UserId == b.UserId).Value) : 
     b.Occurred < expired); 

    IList<B> bs = bQ.ToList(); 
    // ... 
} 

这就产生沿着这些路线的SQL查询：

SELECT 
[Extent1].[ID] AS [ID], 
[Extent1].[OCCURRED] AS [OCCURRED], 
[Extent1].[USERID] AS [USERID], 
FROM [dbo].[B] AS [Extent1] 
OUTER APPLY (SELECT TOP (1) 
    [Extent2].[GROUPNAME] AS [GROUPNAME], 
    [Extent2].[NAME] AS [NAME], 
    [Extent2].[USERID] AS [USERID], 
    [Extent2].[VALUE] AS [VALUE], 
    FROM [dbo].[A] AS [Extent2] 
    WHERE (N'Notifications' = [Extent2].[GROUPNAME]) AND (N'Retention' = [Extent2].[NAME]) AND ([Extent2].[USERID] = [Extent1].[USERID])) AS [Element1] 
    OUTER APPLY (SELECT TOP (1) 
    [Extent3].[GROUPNAME] AS [GROUPNAME], 
    [Extent3].[NAME] AS [NAME], 
    [Extent3].[USERID] AS [USERID], 
    [Extent3].[VALUE] AS [VALUE], 
    FROM [dbo].[A] AS [Extent3] 
    WHERE (N'Notifications' = [Extent3].[GROUPNAME]) AND (N'Retention' = [Extent3].[NAME]) AND ([Extent3].[USERID] = [Extent1].[USERID])) AS [Element2] 
    WHERE (CASE WHEN (EXISTS (SELECT 
    1 AS [C1] 
    FROM [dbo].[A] AS [Extent4] 
    WHERE (N'Notifications' = [Extent4].[GROUPNAME]) AND (N'Retention' = [Extent4].[NAME]) AND ([Extent4].[USERID] = [Extent1].[USERID]) 
)) THEN CASE WHEN (CAST([Extent1].[OCCURRED] AS datetime2) < (DATEADD (hours, -([Element1].[VALUE]), @p__linq__0))) THEN cast(1 as bit) WHEN (NOT (CAST([Extent1].[OCCURRED] AS datetime2) < (DATEADD (hours, -([Element2].[VALUE]), @p__linq__0)))) THEN cast(0 as bit) END WHEN ([Extent1].[OCCURRED] < @p__linq__1) THEN cast(1 as bit) WHEN (NOT ([Extent1].[OCCURRED] < @p__linq__1)) THEN cast(0 as bit) END) = 1 

请注意，我从实际的东西砍死的代码和SQL查询下来，所以这可能不是完美的代表。但我希望它能够得到重点：这看起来有点毛骨悚然，至少在查询重复检查A以匹配组名，用户名和用户名的方式上。但我不是数据库专家。我所知道的是，观察到的执行时间大约为2.5秒。

有没有更好的方法去做这件事？

感谢您的任何意见！

----解决方案----

感谢Gert为此。

代码的查询弄成这个样子：

var bQ = 
    from b in context.Bs 
    let offset = aQ.FirstOrDefault(a => a.UserId == b.UserId) 
    let expiration = (null != offset) ? EntityFunctions.AddHours(now, -offset.Value) : expired 
    where b.Occurred < expiration 
    select b; 

这几乎是什么格特建议。新的SQL查询看起来是这样的：

SELECT 
[Extent1].[ID] AS [ID], 
[Extent1].[OCCURRED] AS [OCCURRED], 
[Extent1].[USERID] AS [USERID] 
FROM [dbo].[B] AS [Extent1] 
OUTER APPLY (SELECT TOP (1) 
    [Extent2].[GROUPNAME] AS [GROUPNAME], 
    [Extent2].[NAME] AS [NAME], 
    [Extent2].[USERID] AS [USERID], 
    [Extent2].[VALUE] AS [VALUE] 
    FROM [dbo].[A] AS [Extent2] 
    WHERE (N'Notifications' = [Extent2].[GROUPNAME]) AND (N'Retention' = [Extent2].[NAME]) AND ([Extent2].[USERID] = [Extent1].[USERID])) AS [Element1] 
    WHERE CAST([Extent1].[OCCURRED] AS datetime2) < (CASE WHEN ([Element1].[ID] IS NOT NULL) THEN DATEADD (hour, -([Element1].[VALUE]), @p__linq__0) ELSE @p__linq__1 END) 

Answer 1

我认为这将查询A表只有一次：

from b in context.Bs 
    let offset = aQ.FirstOrDefault(a => a.UserId == b.UserId).Value 
    let dd = offset.HasValue 
     ? EntityFunctions.AddHours(now, -offset) 
     : expired 
where b.Occurred < dd