I was recently cleaning up unused indexes and I found quite a few. These were mostly part of join models from which a reverse association wasn’t required. Consider the following example:

class ShoppingCart < ActiveRecord::Base
  has_many :shopping_cart_products
  has_many :products, through: :shopping_cart_products

class ShoppingCartProduct < ActiveRecord::Base
  belongs_to :shopping_cart
  belongs_to :product

class Product < ActiveRecord::Base

A very basic design for shopping carts containing products.

The ShoppingCartProduct model has been created using Rails’ model generator, which creates the model class with associations, as well as a migration:

bin/rails g model CartProduct quantity:integer{1} user:belongs_to product:belongs_to –no-test-framework

class CreateShoppingCartProducts < ActiveRecord::Migration
  def change
    create_table :shopping_cart_products do |t|
      t.integer :quantity, limit: 1, null: false
      t.belongs_to :shopping_cart, index: true, foreign_key: true
      t.belongs_to :product, index: true, foreign_key: true

      t.timestamps null: false

As a good practice, the quantity column has been set to disallow null values. Note the two indexes on shopping_cart_products which were created by default.

It doesn’t actually make a lot of sense to ever display which shopping carts a specific product is in. That means it’s not necessary to be able to query a ShoppingCartProduct by its product_id, thus that index is unused. Since Rails generated the model and migration for us, it snuck in really quietly.

An unused index is a waste of database resources. It takes up storage space, and it slows down UPDATEs and INSERTs, in this case on the shopping_cart_products table.

Getting rid of this unused index is easy:

class RemoveIndexShoppingCartProductsOnProductId < ActiveRecord::Migration
  def up
    remove_index(:shopping_cart_products, :product_id)

  def down
    add_index(:shopping_cart_products, :product_id)

Always be conscious about adding database indexes. Only add one if you’re really sure you need it.

As application requirements may change over time, so may the need for indexes. Use database tools to regularly monitor for potentially unused indexes. Be sure to monitor it on the production environment to get the best results.