Rails -- database index necessary for :id attribute?

Question

So as I was following the Ruby on Rails Tutorial by Michael Hartl I noticed that in the users table we added a unique index for the :email attribute to improve the efficiency of the find method so it doesn't search row by row. So far we have been searching using both find_by_email and find_by_id depending on the case. Yet we never set up an index for the :id attribute. Is :id automatically indexed because it is by default unique and sequential in nature? Or is this not the case and should I add an index for :id searching?

Answer 1

Most databases (sqlite included, which is the default db in RoR) automatically index the primary key, which with Rails Migrations is :id by default.