Add an index on message sender.
user
```ruby
class AddMessageSenderIndex < ActiveRecord::Migration
  def self.up
    add_index :messages, [:from_user_id], :name=> "messages_from_user_id_idx"
  end

  def self.down
    drop_index :messages, :name=> "messages_from_user_id_idx"
  end
end
```
assistant