A migration to improve Delayed Job performance when runned with Delayed::Worker.destroy_failed_jobs = false.

improve_delayed_jobs_index.rb

class ImproveDelayedJobsIndex < ActiveRecord::Migration
  def self.up
    remove_index :delayed_jobs, :name => "delayed_jobs_priority"
    add_index :delayed_jobs, [:failed_at, :priority, :run_at], :name => 'delayed_jobs_priority'
  end

  def self.down
    remove_index :delayed_jobs, :name => "delayed_jobs_priority"
    add_index :delayed_jobs, [:priority, :run_at], :name => 'delayed_jobs_priority'
  end
end