Used internally by adapters.
To define an index in a migration, see Ecto.Migration.index/3
Ecto.Migration.index/3
t() :: %Ecto.Migration.Index{columns: [atom | String.t], comment: String.t | nil, concurrently: boolean, name: atom, options: String.t, prefix: atom, table: String.t, unique: boolean, using: atom | String.t, where: atom | String.t}