Why the Left Outer join?

问题

weird one. (Probably not weird, at all)

I have 3 objects, Employee, Rota and Department.

public class Employee
{
    public int Id { get; set; }
    public String Name { get; set; }
    public virtual Department Department { get; set; }
}

internal class EmployeeMapping : EntityTypeConfiguration<Employee>
{
    public EmployeeMapping()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasColumnName("UserId");

        HasRequired<Department>(a => a.Department).WithOptional().Map(a => a.MapKey("DepartmentId"));
    }
}

public class Department
{
    public int Id { get; set; }
    public String Name { get; set; }
}

internal class DepartmentMapping : EntityTypeConfiguration<Department>
{
    public DepartmentMapping()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasColumnName("DepartmentId");
    }
}

public class Rota
{
    public int Id { get; set; }
    public virtual Employee Employee { get; set; }
    public virtual Department Department { get; set; }
}

internal class RotaMapping : EntityTypeConfiguration<Rota>
{
    public RotaMapping()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasColumnName("RotaId");

        HasOptional<Employee>(a => a.Employee).WithOptionalDependent().Map(a => a.MapKey("EmployeeId"));
        HasOptional<Department>(a => a.Department).WithOptionalDependent().Map(a => a.MapKey("DepartmentId"));
    }
}

Not complicated, at all really. Rota can have an Employee and/or a Department assigned to it, all of this is configured using Fluent. All of my associations are correct (the schema is perfect), however I have a weird oddity.

When I do a myContext.Departments.FirstOrDefault() and have a look at the SQL Generated, there is a LEFT OUTER JOIN on Employee & Rota. Why is this there?
I don't want it to do this. Maybe my Fluent mappings are incorrect? I've tried all sorts, but can't seem to figure it out. I would understand it if I want a Rota object, that would join on the Department. But not the other way around!

If I do myContext.Departments.AsNoTracking().FirstOrDefault() it doesn't do the LEFT OUTER JOIN's.

Any ideas guys?

Cheers, D

回答1:

The reason is incorrect mapping. It looks correct but it is not. Use these instead:

internal class EmployeeMapping : EntityTypeConfiguration<Employee>
{
    public EmployeeMapping()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasColumnName("UserId");

        HasRequired<Department>(a => a.Department).WithMany()
                                                  .Map(a => a.MapKey("DepartmentId"));
    }
}

internal class RotaMapping : EntityTypeConfiguration<Rota>
{
    public RotaMapping()
    {
        HasKey(a => a.Id);
        Property(a => a.Id).HasColumnName("RotaId");

        HasOptional<Employee>(a => a.Employee).WithMany()
                                              .Map(a => a.MapKey("EmployeeId"));
        HasOptional<Department>(a => a.Department).WithMany()
                                                  .Map(a => a.MapKey("DepartmentId"));
    }
}

Your mapping is correctly interpreted when creating database and database looks correct but EF thinks that you map all relations as one-to-one. That confuse EF and it will generate queries used for one-to-one to create internal entity references. These left joins are necessary for one-to-one relation when you tell EF that dependent entities are optional - EF doesn't know if they exist unless it loads their keys.

回答2:

I am by no means an expert and I am just guessing here. But could you try the query the way entity framework is doing it and then try it the way you would ideally like to see the query and post the time differentials. Maybe the entity framework is on to something here, but I doubt it.

回答3:

I have theory. The left outer join goes away when you turn off change tracking. Also Employee and Rota have references to Department.

So my theory is that with change tracking on the Entity Framework tries to load all entities with a reference to the department in case it has to cascade an update to department.

In other words it thinks that a change to a Department could cause a change to the Employee or Rota referencing the department so it loads everything just in case.

来源：https://stackoverflow.com/questions/6413063/why-the-left-outer-join