Optimizing LINQ To Entities query - entity-framework

Question

OK, let's say that I have two entities, A and B. For each user, there may be 0 or more A, with a unique combination of UserId (foreign key), GroupName, and Name properties. Entity A also has an "Value" property. Also for each user, there may be 0 or more B, with a UserID (again, a foreign key) and an "Occurred" property.
My task is to find all the B which have an Occurred property of more than 7 days ago OR have an Occurred property more than now - the number of hours in a particular A property. This code seems to work perfectly:
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();
// ...
}
This produces a SQL query along these lines:
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
Please note that I've hacked the code and SQL query down from the actual stuff, so this may not be the perfect representation. But I hope it gets the point across: this looks a little hairy, at least in the way that the query is repeatedly checking A for matching groupname, name, and user. But I'm no Database expert. What I do know is that one observed execution ran roughly 2.5 seconds.
Is there a better way of going about this?
Thanks for any input!
---- SOLUTION ----
Thanks to Gert for this.
The code's query ended up like this:
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;
Which is almost exactly what Gert suggested. The new SQL query looks like this:
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

I think that this will query the A table only once:
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