Remove redundant join for table splitting

Start with the table-splitting model and mapping in the reproduction, then compare the SQL generated for the queries with and without Include. Trace the table-splitting include query path and determine whether the self-join can be omitted without losing the cancellation fields. Done means the include query loads the mapped columns without the redundant self-join and preserves the expected results.

When using table splitting, EF Core is joining the table with itself to get the second entity field. It seems that was not the case in EF6 (see Select scenario, test C).

The objective is not to load all the columns of a table each time, but group them by functionality and load them when they are going to be used.

Am I doing anything wrong? Is there anything I can do better?

Steps to reproduce

Model

public class PurchaseOrder
{
    public int Id { get; set; }
    public string Status { get; set; }
    public PurchaseOrderCancellation Cancellation { get; set; }
}

public class PurchaseOrderCancellation
{
    public string CancelReason { get; set; }
    public DateTime? CancelDate { get; set; }
}

Mapping

class PurchaseOrderConfig : IEntityTypeConfiguration<PurchaseOrder>
{
    public void Configure(EntityTypeBuilder<PurchaseOrder> builder)
    {
        builder.ToTable("purchase_order");
        builder.HasKey(x => x.Id);
        builder.Property(x => x.Id).HasColumnName("id").ForSqlServerUseSequenceHiLo();
        builder.Property(x => x.Status).HasColumnName("status");
        builder.HasOne(x => x.Cancellation).WithOne().HasForeignKey<PurchaseOrder>(x => x.Id);
    }
}

class PurchaseOrderCancellationConfig : IEntityTypeConfiguration<PurchaseOrderCancellation>
{
    public void Configure(EntityTypeBuilder<PurchaseOrderCancellation> builder)
    {
        builder.ToTable("purchase_order");
        builder.Property(x => x.CancelReason).HasColumnName("cancel_reason").HasMaxLength(500);
        builder.Property(x => x.CancelDate).HasColumnName("cancel_date").HasColumnType("datetime2");
        builder.Property<int>("Id").HasColumnName("id").ForSqlServerUseSequenceHiLo();
        builder.HasKey("Id");
    }
}

Queries:

Without include

var query1 = ctx.Set<PurchaseOrder>()
                .Where(x => x.Id == 1)
                .ToArray();

SELECT [x].[id], [x].[status]
FROM [purchase_order] AS [x]
WHERE [x].[id] = 1

With include

var query2 = ctx.Set<PurchaseOrder>()
                .Include(x => x.Cancellation)
                .Where(x => x.Id == 1);

SELECT [x].[id], [x].[status], [x.Cancellation].[id], [x.Cancellation].[cancel_date], [x.Cancellation].[cancel_reason]
FROM [purchase_order] AS [x]
INNER JOIN [purchase_order] AS [x.Cancellation] ON [x].[id] = [x.Cancellation].[id]
WHERE [x].[id] = 1

Further technical details

EF Core version: 2.0
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 10
IDE: VS2017 15.3