实体框架多对多关系包括极其缓慢

Question

I have an Entity Framework 4 model, with 2 entities containing many-to-many relationship, so 3 tables, [Q], [P] and [Q2P]-cross table. Running code like:

context.Q.Include("P");

Results in long time wait (I waited like 5 mins then aborted it). Then I checked SQL generated and found this:

SELECT *
FROM ( SELECT *
    FROM  [Q] AS [Extent1]
    LEFT OUTER JOIN  (SELECT *, CASE WHEN ([Join1].[Id] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2]
        FROM  [Q_P] AS [Extent2]
        INNER JOIN [P] AS [Extent3] ON [Extent3].[Id] = [Extent2].[Id] ) AS [Join1] ON [Extent1].[Id] = [Join1].[Id]
)  AS [Project1]
ORDER BY [Project1].[Id] ASC, [Project1].[C2] ASC

I can't hide my suprise, WTF is this? The usual many-to-many SQL query

select * from [q2p]
join [q] on qId=q.Id
join [p] on pId=p.Id

executes in less than 1ms, while EF query executes forever.

Answer 1

Yep, that's not a secret that it takes long, please vote on the connection I opened about a year ago.

However, 5 minutes is something it's definitely not supposed to take.

Try separating the execution from the query generation, and use ToTraceString to see how long does it take to determine query generation time.

Eager-loading is not the great deal in current version, they said they're planning toreduce the performance cost in future.

Anyway, what you could do is use Stored Procedures or create your own ObjectQueries.

See:
- http://msdn.microsoft.com/en-us/library/bb896241.aspx
- http://msdn.microsoft.com/en-us/library/bb896238.aspx

Answer 2

我已切换到Nhibernate。