Linq-to-SQL left join on left join/multiple left joins in one Linq-to-SQL statement

问题

I'm trying to rewrite SQL procedure to Linq, it all went well and works fine, as long as it works on small data set. I couldn't really find answer to this anywhere. Thing is, I have 3 joins in the query, 2 are left joins and 1 is inner join, they all join to each other/like a tree. Below you can see SQL procedure:

SELECT ...
    FROM sprawa s (NOLOCK) 
        LEFT JOIN strona st (NOLOCK) on s.ident = st.id_sprawy
        INNER JOIN stan_szczegoly ss (NOLOCK) on s.kod_stanu = ss.kod_stanu
        LEFT JOIN broni b (NOLOCK) on b.id_strony = st.ident

What I'd like to ask you is a way to translate this to Linq. For now I have this:

var queryOne = from s in db.sprawa
               join st in db.strona on s.ident equals st.id_sprawy into tmp1
               from st2 in tmp1.DefaultIfEmpty()
               join ss in db.stan_szczegoly on s.kod_stanu equals ss.kod_stanu
               join b in db.broni on st2.ident equals b.id_strony into tmp2
               from b2 in tmp2.DefaultIfEmpty()
               select new { };

Seems alright, but when checked with SQL Profiler, query that is sent to database looks like that:

SELECT ... FROM    [dbo].[sprawa] AS [Extent1] 
           LEFT OUTER JOIN [dbo].[strona] AS [Extent2] 
                ON [Extent1].[ident] = [Extent2].[id_sprawy]    
           INNER JOIN [dbo].[stan_szczegoly] AS [Extent3] 
                ON [Extent1].[kod_stanu] = [Extent3].[kod_stanu]    
           INNER JOIN [dbo].[broni] AS [Extent4] 
                ON ([Extent2].[ident] = [Extent4].[id_strony]) OR 
                (([Extent2].[ident] IS NULL) AND ([Extent4].[id_strony] IS NULL))

As you can see both SQL queries are bit different. Effect is the same, but latter works incomparably slower (less than a second to over 30 minutes). There's also a union made, but it shouldn't be the problem. If asked for I'll paste code for it.

I'd be grateful for any advice on how to better the performance of my Linq statement or how to write it in a way that is translated properly.

回答1:

I guess I found the solution:

var queryOne = from s in db.sprawa
               join st in db.strona on s.ident equals st.id_sprawy into tmp1
               where tmp1.Any()
               from st2 in tmp1.DefaultIfEmpty()
               join ss in db.stan_szczegoly on s.kod_stanu equals ss.kod_stanu
               join b in db.broni on st2.ident equals b.id_strony into tmp2
               where tmp2.Any()
               from b2 in tmp2.DefaultIfEmpty()
               select new { };

In other words where table.Any() after each into table statement. It doesn't make translation any better but has sped up execution time from nearly 30minutes(!) to about 5 seconds.

This has to be used carefully though, because it MAY lead to losing some records in result set.

来源：https://stackoverflow.com/questions/17145640/linq-to-sql-left-join-on-left-join-multiple-left-joins-in-one-linq-to-sql-statem