How to create a Clustered Index with Entity Framework Core

Question

From EF6.1, we have a way of specifying a clustered index on a property

public class Person 
{
  [Index(IsClustered = true, IsUnique = true)]
  public long U         


        

Answer 1

In the absence of built-in support, you can use a custom attribute of your own to annotate model properties and apply in OnModelCreating():

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    foreach (var entity in modelBuilder.Model.GetEntityTypes())
    {
        foreach (var prop in entity.GetProperties())
        {
            var attr = prop.PropertyInfo.GetCustomAttribute<IndexAttribute>();
            if (attr != null)
            {
                var index = entity.AddIndex(prop);
                index.IsUnique = attr.IsUnique;
                index.SqlServer().IsClustered = attr.IsClustered;
            }
        }
    }
}

With a simple marker attribute class:

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class IndexAttribute : Attribute
{
    public bool IsUnique { get; set; } 
    public bool IsClustered { get; set; } 
}

Then in your model class, just add the attribute to create a secondary index:

public class User
{
    public int UserId { get; set; }
    [Index(IsUnique = true, IsClustered = true)]
    public string Nickname { get; set; }
}

Answer 2

Or this works too, say if you wanted to cluster by age...

            modelBuilder
            .Entity<Person>()
            .Property(t => t.Age)
            .HasColumnAnnotation("Index", new IndexAnnotation(new IndexAttribute() { IsClustered = true}));

Answer 3

From the current EF Core documentation - Indexes section:

Data Annotations

Indexes can not be created using data annotations.

But for sure you can specify that via Fluent API (note the extension methods having ForSqlServer prefix which seem to denote SqlServer specific features):

modelBuilder.Entity<Person>()
    .HasIndex(e => e.UserName)
    .IsUnique()
    .ForSqlServerIsClustered();

Answer 4

For EF Core 3.0+ You can now use IsClustered:

modelBuilder.Entity<Person>()
.HasIndex(e => e.UserName)
.IsUnique()
.IsClustered();

.ForSqlServerIsClustered() is now marked as obsolete.

Also be aware that if you have a Primary Key on the table you may also need to explicitly remove the clustering on it before you add the clustering on your Username:

modelBuilder.Entity<Person>()
.HasKey(e => e.PersonId)
.IsClustered(false);

modelBuilder.Entity<Person>()
.HasIndex(e => e.UserName)
.IsUnique()
.IsClustered();