实体框架按多个相关实体计数顺序 - 效率

Question

So I have a Topic which has these related entities.

 - List<Posts>
 - List<Votes>
 - List<Views>

I have the following query. Where I want to pull out and order popular Topics based on the count of 3 related entities over a specific date period.

var topics = _context.Topic
.OrderByDescending(x => x.Posts.Count(c => c.DateCreated >= from && c.DateCreated <= to))
 .ThenByDescending(x => x.Votes.Count(c => c.DateCreated >= from && c.DateCreated <= to))
.ThenByDescending(x => x.Views.Count(c => c.DateCreated >= from && c.DateCreated <= to))
.Take(amountToShow)
.ToList();

I'm looking for the most efficient query for doing the above? Is what I am doing the best way to do this with EntityFramework? Or am I missing something?

Any help appreciated.

Answer 1

If you put your above code into LINQPad , or check it with the profiler, you will see that it will probably generate something like the following SQL:

SELECT TOP @amountToShow [t0].[id] --and additional columns
FROM [Topic] AS [t0]
ORDER BY (
    SELECT COUNT(*)
    FROM [Posts] AS [t1]
    WHERE ([t1].DateCreated >= @from AND [t1].DateCreated <= @to) 
        AND ([t1].[topidId] = [t0].[id])
    ) DESC, (
    SELECT COUNT(*)
    FROM [Votes] AS [t2]
    WHERE ([t2].DateCreated >= @from AND [t2].DateCreated <= @to) 
        AND ([t2].[topicId] = [t0].[id])
    ) DESC , (
    SELECT COUNT(*)
    FROM [Views] AS [t3]
    WHERE ([t3].DateCreated >= @from AND [t3].DateCreated <= @to) 
        AND ([t3].[topicId] = [t0].[id])
    ) DESC
GO

You could try rewriting the SQL a bit to GROUP the results of the subqueries and LEFT JOIN them to the original table, which does seem to be about 2x faster in the db itself:

SELECT TOP @amountToShow [t0].[id] --etc
FROM [Topic] AS [t0]
LEFT JOIN 
    (SELECT topicId, COUNT(*) AS num FROM Posts p 
    WHERE [p].DateCreated >= @from AND .DateCreated <= @to 
    GROUP BY topicId) [t1]
ON t0.id = t1.topicId
LEFT JOIN 
    (SELECT topicId, COUNT(*) AS num FROM Votes vo 
    WHERE [vo].DateCreated >= @from AND [vo].DateCreated <= @to 
    GROUP BY topidId) [t2]
ON t0.id = t2.topicId
LEFT JOIN 
    (SELECT topicId, COUNT(*) AS num FROM Views vi 
    WHERE [vi].DateCreated >= @from AND [vi].DateCreated <= @to 
    GROUP BY topicId) [t3]
ON t0.id = t3.topicId
ORDER BY t1.num DESC, t2.num DESC, t3.num DESC

But getting LINQ to generate code like this is iffy at best. Doing LEFT JOIN s are not exactly its strong suit, and using the techniques that are out there for doing so will probably generate SQL that uses CROSS APPLY and/or OUTER APPLY instead, and will likely be as slow or slower than your current code.

If you are that worried about speed, you might consider putting your fine-tuned SQL into a view so that you know that the query being used is the one you want.

Bear in mind, too, that you or someone else will have to come back to this code and maintain it later. Your current linq statement is very straightforward and easy to understand. A complicated query is going to be harder to maintain and will take more work to alter in the future.