Query: Left Join (GroupJoin) always materializes elements resulting in unnecessary data pulling

[qmicron]

Steps to reproduce

public class Budget
{
    public Guid Id { get; set; }
    public Guid? DescriptionTextId { get; set; }
    public decimal PostCost { get; set; }
    public string PostNr { get; set; }
    public decimal PostPrice { get; set; }
    public decimal Quantity { get; set; }
    public decimal UnitCost { get; set; }
    public string UnitName { get; set; }
    public decimal UnitPrice { get; set; }
    public decimal VatAmount { get; set; }
    public decimal VatBasis { get; set; }
    public decimal VatFreeAmount { get; set; }
    public decimal VatFreePercent { get; set; }
    public decimal VatIncluded { get; set; }
    public decimal VatRate { get; set; }

    public Text DescriptionText { get; set; }
}

public class Text
{
    public Guid Id { get; set; }
    public string Plain { get; set; }
    public string Rtf { get; set; }

    public ICollection<Budget> BudgetDescriptionText { get; set; } = new List<Budget>();
}

public class DataContext : DbContext
{
    public DataContext(DbContextOptions options)
        : base(options)
    {
    }

    public DbSet<Budget> Budget => Set<Budget>();
    public DbSet<Text> Text => Set<Text>();

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Budget>(entity =>
        {
            entity.HasKey(e => e.Id);
            entity.Property(e => e.Id).ValueGeneratedNever();
            entity.Property(e => e.DescriptionTextId);
            entity.Property(e => e.PostCost);
            entity.Property(e => e.PostNr).IsRequired().HasMaxLength(1000);
            entity.Property(e => e.PostPrice);
            entity.Property(e => e.Quantity);
            entity.Property(e => e.UnitCost);
            entity.Property(e => e.UnitName).HasMaxLength(100);
            entity.Property(e => e.UnitPrice);
            entity.Property(e => e.VatAmount);
            entity.Property(e => e.VatBasis);
            entity.Property(e => e.VatFreeAmount);
            entity.Property(e => e.VatFreePercent);
            entity.Property(e => e.VatIncluded);
            entity.Property(e => e.VatRate);
            entity.HasOne(e => e.DescriptionText)
                .WithMany(e => e.BudgetDescriptionText).HasForeignKey(e => e.DescriptionTextId);
        });

        modelBuilder.Entity<Text>(entity =>
        {
            entity.HasKey(e => e.Id);
            entity.Property(e => e.Id).ValueGeneratedNever();
            entity.Property(e => e.Plain);
            entity.Property(e => e.Rtf);
        });
    }
}

The issue

We expect that the following query returns values for two columns

var result = context.Budget
    .Select(p => new
    {
        p.PostCost,
        p.DescriptionText.Plain
    })
    .ToArray();

but in reality, EF generates SQL query that returns all columns.

SELECT 
    [p].[Id], 
    [p].[DescriptionTextId], 
    [p].[PostCost], 
    [p].[PostNr], 
    [p].[PostPrice], 
    [p].[Quantity], 
    [p].[UnitCost], 
    [p].[UnitName], 
    [p].[UnitPrice], 
    [p].[VatAmount], 
    [p].[VatBasis], 
    [p].[VatFreeAmount], 
    [p].[VatFreePercent], 
    [p].[VatIncluded], 
    [p].[VatRate], 
    [p.DescriptionText].[Id], 
    [p.DescriptionText].[Plain], 
    [p.DescriptionText].[Rtf]
FROM 
    [Budget] AS [p]
    LEFT JOIN [Text] AS [p.DescriptionText] ON [p].[DescriptionTextId] = [p.DescriptionText].[Id]
ORDER BY 
    [p].[DescriptionTextId]

Further technical details

EF Core version: 1.0.1
Operating system: Windows 10
Visual Studio version: VS 2015

[smitpatel]

This is due to requiring materialization of both the sides in group join.
DescriptionText is optional navigation hence it will generate a group join query. In group join query changes in the outer elements need to be recorded (bug #6360). This was fixed in #6387 by pulling all columns to figure out the changes in outer element.

[rowanmiller]

@maumar to check if we already have work items covering this

[maumar]

Another commit related to this issue: e514b49 (fix to #4858)

[tuespetre]

#7543 covers this.

[maumar]

Fixed in 03a990c