Question

This question takes no time at all to execute, but if I uncomment the last line it takes 5 seconds. How could this affect the query that much? It’s almost the exact same query.

return db.Parties.Where(predicate).Select(p => new SearchResult
{
    adressPerson = p.Contacts.SingleOrDefault(m => m.contact_type == "H").streetname,
    //adressOrg = p.Contacts.SingleOrDefault(m => m.contact_type == "W").streetname
});  

How do I locate the problem?

Here’s what the predicate looks like:

    predicate = predicate.Or(p => p.surname.EndsWith(keyword));
    predicate = predicate.Or(p => p.lastname.EndsWith(keyword));
    predicate = predicate.Or(p => p.organisation.EndsWith(keyword));
    predicate = predicate.Or(p => p.Contacts.Any(c => c.streetname.EndsWith(keyword)));
    predicate = predicate.Or(p => p.Memberships.Any(m => m.Congregation.congregation_name.EndsWith(keyword)));
    predicate = predicate.Or(p => p.PartyCategories.Any(m => m.Category.category_name.EndsWith(keyword) || m.Category.category_code.EndsWith(keyword)));  

I know it’s stupid to post a lot of code but here’s the SQL-query anyway. Any clues?

SELECT (
    SELECT [t6].[streetname]
    FROM [dbo].[Contact] AS [t6]
    WHERE ([t6].[contact_type] = @p7) AND ([t6].[party_id] = [t0].[party_id])
    ) AS [adressPerson], (
    //SELECT [t7].[streetname]
    //FROM [dbo].[Contact] AS [t7]
    //WHERE ([t7].[contact_type] = @p8) AND ([t7].[party_id] = [t0].[party_id])
    //) AS [adressOrg]
FROM [dbo].[Party] AS [t0]
WHERE ([t0].[surname] LIKE @p0) OR ([t0].[lastname] LIKE @p1) OR ([t0].[organisation] LIKE @p2) OR (EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [dbo].[Contact] AS [t1]
    WHERE ([t1].[streetname] LIKE @p3) AND ([t1].[party_id] = [t0].[party_id])
    )) OR (EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [dbo].[Membership] AS [t2]
    LEFT OUTER JOIN [dbo].[Congregation] AS [t3] ON [t3].[congregation_id] = [t2].[congregation_id]
    WHERE ([t3].[congregation_name] LIKE @p4) AND ([t2].[party_id] = [t0].[party_id])
    )) OR (EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [dbo].[PartyCategories] AS [t4]
    INNER JOIN [dbo].[Category] AS [t5] ON [t5].[category_id] = [t4].[category_id]
    WHERE (([t5].[category_name] LIKE @p5) OR ([t5].[category_code] LIKE @p6)) AND ([t4].[party_id] = [t0].[party_id])
))

SOLVED
Turned out that the INNER JOIN in the last select statement didn’t work. So I changed the last line of the predicate to:

predicate = predicate.Or(p => p.PartyCategories
    .GroupJoin(db.Categories, b => b.category_id, c => c.category_id, (b, c) => new { b, c })
    .SelectMany(c => c.c.DefaultIfEmpty(), (c, b) => new { c.c, b })
    .Any(m => m.b.category_name.EndsWith(keyword) || m.b.category_code.EndsWith(keyword)));

Answer 1

How do I locate the problem?

As always, the way to find out is to look at the generated SQL, and run it through SQL profiler.

My guess is that it changes from a simple inner join when you’re only interested in a single contact type into something rather more complicated, or possibly even an “N+1-selects” problem. Anyway, looking into the SQL should make it a lot clearer.