Question

I know that there are some questions about this already, most relate to either old issues which were resolved or multiple tables. This question is not covered in any of the other ‘left outer join’ issues I saw, I get an INNER JOIN and LEFT OUTER JOIN to the same table at the same query.

The table outline:

Users: id (PK)
       Name (VARCHAR) 
       ProfileImageUri (VARCHAR)
Locations: id (PK)
LocationBPNTips: id (PK)
                 TipText (VARCHAR)
                 CreatedAt (Datetime)
                 UserId (int) (FK to User.id, navigation property is called User)
                 LocationId (int) (FK to Location.id)

(there is more, but it is not relevant 🙂 )

In my scenario, I am performing a query to a referenced table via projection and I get an extra left outer join, this is what I run (the commented parts are irrelevant to the problem, commented out for cleaner SQL, EF does the sorting right (even better than I imagined 🙂 ) ):

LocationBPNTips
     .Where(t => t.LocationId == 33)
     //.OrderByDescending(t => intList.Contains(t.UserId))
     //.ThenByDescending(t => t.CreatedAt)
     .Select(tip => new LocationTipOutput
     {
             CreatedAt = tip.CreatedAt,
             Text = tip.TipText,
             LocationId = tip.LocationId,
             OwnerName = tip.User.Name,
             OwnerPhoto = tip.User.ProfileImageUri
     }).ToList();

And this is is the generated SQL

SELECT 
[Extent1].[LocationId] AS [LocationId], 
[Extent1].[CreatedAt] AS [CreatedAt], 
[Extent1].[TipText] AS [TipText], 
[Extent2].[Name] AS [Name], 
[Extent3].[ProfileImageUri] AS [ProfileImageUri]
FROM   [dbo].[LocationBPNTips] AS [Extent1]
INNER JOIN [dbo].[Users] AS [Extent2] ON [Extent1].[UserId] = [Extent2].[Id]
LEFT OUTER JOIN [dbo].[Users] AS [Extent3] ON [Extent1].[UserId] = [Extent3].[Id]
WHERE 33 = [Extent1].[LocationId]

As you can see, the LEFT OUTER JOIN is done on the same table of the INNER JOIN

I think the optimal code will be (note, I renamed Extent3 to Extent2 manually, and added the comment. this was not generated by EF!!) – with my current data, this runs about 22% faster (with the sorting, this % should be higher without the sort) as no need for an extra join..

SELECT 
[Extent1].[LocationId] AS [LocationId], 
[Extent1].[CreatedAt] AS [CreatedAt], 
[Extent1].[TipText] AS [TipText], 
[Extent2].[Name] AS [Name], 
[Extent2].[ProfileImageUri] AS [ProfileImageUri]
FROM   [dbo].[LocationBPNTips] AS [Extent1]
INNER JOIN [dbo].[Users] AS [Extent2] ON [Extent1].[UserId] = [Extent2].[Id]
--LEFT OUTER JOIN [dbo].[Users] AS [Extent3] ON [Extent1].[UserId] = [Extent3].[Id]
WHERE 33 = [Extent1].[LocationId]

The different queries I have tried (the projection is into an anonymous type in these):

LocationBPNTips
     .Where(t => t.LocationId == 33)
     //.OrderByDescending(t => intList.Contains(t.UserId))
     //.ThenByDescending(t => t.CreatedAt)
     .Select(tip => new 
     {
             CreatedAt = tip.CreatedAt,
             Text = tip.TipText,
             LocationId = tip.LocationId,
             OwnerName = tip.User,
             OwnerPhoto = tip.User
     }).ToList()

SQL output was messed up, it selected the entire user table twice in the same format as above, inner then left outer. I think that I can see in theory why this happens for this case, because I asked for the data twice – although it could have been done in memory and not by the SQL with an extra join – but in my case I did not ask for data twice, I asked for different columns only once. I did this test to see if the double join is consistent.

I also tried running:

LocationBPNTips
    .Where(t => t.LocationId == 33)
    .Select(tip => new 
    {
            CreatedAt = tip.CreatedAt,
            Text = tip.TipText,
            LocationId = tip.LocationId,
            OwnerName = tip.User.Name
    }).ToList()

This one returned clean, single inner join as expected, but it is not what I am trying to do

So the question is: Is this a bug? am I using the EF incorrectly?

Answer 1

I’ve seen similar problem already. We can call it bug or feature. Simply EF query generation is far from ideal. ADO.NET team fixes some problems with every major release. I don’t have June CTP 2011 currently installed to verify if it also happens in the first preview of the next version.

Edit:

According to this answer similar issue was fixed in June CTP 2011.