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。此外，对于每个用户，可能有0个或更多B，具有UserID（再次，外键）和“发生”属性。

我的任务是找到所有发生超过7天的发生属性的B或者比现在更多的发生的属性 - 特定A属性中的小时数。这段代码似乎完美无缺：

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();
  // ...
}

这会产生一个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，我已经从实际内容中删除了代码和SQL查询，因此这可能不是完美的表示。但是我希望它能得到重点：这看起来有点毛茸茸，至少在查询重复检查A以匹配组名，名称和用户的方式。但我不是数据库专家。我所知道的是，观察到的一次执行大约需要2.5秒。

有没有更好的方法来解决这个问题？

感谢您的任何意见！

----解决方案----

感谢Gert。

代码的查询最终结果如下：

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;

这几乎正是格特所建议的。新的SQL查询如下所示：

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

我认为这只会查询A表一次：

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

优化LINQ To Entities查询

1 个答案: