I am trying to write a LINQ query, just some inner joins and one  left outer join. So when I do the left outer join for classes something like this, its too expensive and I am getting a timeout error.

from p in context.Professors
join d in context.Departments
join c in context.Classes on c.ClassId equals p.ClassId into cs
from c in cs.DefaulIfEmpty()

But if I do it something like this, it works perfectly fine, and not expensive, no timeouts:

from p in context.Professors
join d in context.Departments
from c in context.Classes.Where(x=>.ClassId == p.ClassId).DefaultIfEmpty()