- Looking up a row by an indexed column provides O(1) access to the data.
- When creating a column in a database, it is important to consider whether we will need to find records by that column so that we can avoid a potential full-table scan.
- Any foreign key column should additionally have an index.
# Add an index on the email column of the users table. $ rails generate migration add_index_to_users_email
# The migration for adding an index on the email column of the users table and # enforcing email uniqueness. # Note: The index by itself doesn’t enforce uniqueness. # (The option `unique: true` does.) # https://www.railstutorial.org/book/modeling_users#code-email_uniqueness_index class AddIndexToUsersEmail < ActiveRecord::Migration def change add_index( :users, :email, unique: true ) end end
# NOTE: When you tell Rails something like `belongs_to :photo`, Rails does not # update the database to index `user.photo_id`. # This way, each direction of our user and photo lookup is fast and easy. def up add_index( :users, :username ) add_index( :users, :photo_id ) end