add_index to Data Model - Ruby on Rails Tutorial

Question

I am just a bit stuck on this code found in the Ruby on Rails Tutorial.org. What exactly does the add_index part of it do? Why are there 3 lines for this?

    class CreateRelationships < ActiveRecord::Migration
  def change
    create_table :relationships do |t|
      t.integer :follower_id
      t.integer :followed_id

      t.timestamps
    end

    add_index :relationships, :follower_id
    add_index :relationships, :followed_id
    add_index :relationships, [:follower_id, :followed_id], unique: true
  end
end

Answer 1

A database index is a data structure that improves the speed of operations in a table. Indexes can be created using one or more columns, providing the basis for both rapid random lookups and efficient ordering of access to records. - TutorialPoint

Basically Index used to speed up the query.

In the example

add_index :relationships, :follower_id
add_index :relationships, :followed_id

index is created for follower_id and followed_id column which will speed up the query looking for follower_id OR followed_id. It does not enforce any other constraints on your column like UNIQUE. So they can have identical values

Here

add_index :relationships, [:follower_id, :followed_id], unique: true

the process is same as above with a constraint that follower_id AND followed_id combinations should be distinct. An Error will be thrown if you try to duplicate identical combined values for these columns in multiple rows.