EF Where(x => x.ColumnVal == 1) vs FirstOrDefault(x => x.Column == 1)

Question

I had a LINQ query that loads a hierarchy of objects like the following.

Query #1

var result = db.Orders
               .Include(\"C         


        

Answer 1

I found the culprit. It's the SQL query generated by Entity Framework.
I have a complicated Schema with a lot of many-to-many relationships.

Entity Framework was generating a 32,000 line long SQL string :'(
Now, I am changing my code to load the hierarchy manually for some part.

Please let me know if anyone knows some good articles to read about Eager Loading and Many-to-Many relationships.

Answer 2

I think best would be to use ...Where(condition).Take(1).FirstOrDefault() because Take(1) can be easily translated to SQL as a TOP clause. Anybody with me?

Answer 3

No, they both should result in a same SQL query when being executed. You can prove it by looking into SQL Profiler and see what is the exact SQL being submitted from EF in both cases. Your performance optimization should have been caused by some other factors. Here is why:

Include method returns an ObjectQuery<T>:

public class ObjectQuery<T> : ObjectQuery, IOrderedQueryable<T>, 
                              IQueryable<T>, IEnumerable<T>, 
                              IOrderedQueryable, IQueryable, 
                              IEnumerable, IListSource

Which means its FirstOrDefault method comes with 2 overloads:

// Defined by Enumerable:
FirstOrDefault(Func<T, Boolean>)

// Defined by Queryable:
FirstOrDefault(Expression<Func<T, Boolean>>)

When you code .FirstOrDefault(x => x.Customer.CustomerId == 1 compiler will go into a process called Overload Resolution to infer the type of the lambda expression x => x.Customer.CustomerId == 1 since it is convertible to the type of both overload's parameter types.
Compiler will use an algorithm (that I am still trying to find in C# Language Specification!), figure out that converting the lambda to the Expression<Func<T, Boolean> is a better conversion than to Func<T, Boolean> so pick the IQueryable overload.
Therefore, you'll see the predicate in the generated SQL when observing it in the SQL Profiler.