canvas-lms/db/migrate/20200422173259_add_user_course_id_submissions_index.rb

class AddUserCourseIdSubmissionsIndex < ActiveRecord::Migration[5.2]
  tag :predeploy
  disable_ddl_transaction!

  def change
    add_index :submissions, [:user_id, :course_id], :algorithm => :concurrently, if_not_exists: true
  end
end