The column of an item's position, in a migration: a text compared byte by byte, as position keys must be (see Narabe.Position).

defmodule MyApp.Repo.Migrations.CreateCards do
  use Ecto.Migration
  import Narabe.Migration

  def change do
    create table(:cards) do
      add :title, :string
      add :column_id, references(:columns)
      position()
    end

    create index(:cards, [:column_id, :position])
  end
end

The column is :string, not null when you say so (null: false), with the collation that compares bytes: "C" on Postgres, utf8mb4_bin on MySQL; SQLite compares bytes already. For a column that already exists, alter_position/2 changes its collation.

Index the position after the item's scope (its list, column or parent): a list is read in its order with that index.