Performance regression of ActiveRecord preloading :has_many through associations | Major performance regression when preloading has_many_through association · Issue #12537 · rails/rails https://github.com/rails/rails/issues/12537#issuecomment-55462750

bechmark.rb

begin
  class CreateModels < ActiveRecord::Migration
    def change
      create_table :items do |t|
      end

      create_table :events do |t|
        t.belongs_to :item
      end

      create_table :event_people do |t|
        t.belongs_to :event
        t.belongs_to :person
      end

      create_table :people do |t|
      end
    end
  end

  class Item < ActiveRecord::Base
    has_many :events
    has_many :event_people, :through => :events
    has_many :people, :through => :events
  end

  class Event < ActiveRecord::Base
    belongs_to :item
    has_many :event_people
    has_many :people, :through => :event_people
  end

  class EventPerson < ActiveRecord::Base
    belongs_to :event
    belongs_to :person
  end

  class Person < ActiveRecord::Base
  end

  CreateModels.new.change

  puts "Populating Database"
  1000.times do
    Item.create
  end

  Item.all.each_with_index do |item|
    event = Event.new
    event.people.build
    event.people.build
    item.events << event
  end
rescue => e
  # Tables already exist
  puts e.message
ensure
  require 'benchmark'
  Benchmark.benchmark("#{Rails.version} - #{ActiveRecord::Base.configurations[Rails.env]['adapter']}\n", 40) do |x|
    x.report('limit 100 has_many') { 1.times { Item.preload(:events).limit(100).order(:id).to_a } }
    x.report('limit 100 has_many_through') { 1.times { Item.preload(:event_people).limit(100).order(:id).to_a } }
    x.report('limit 100 double has_many_through') { 1.times { Item.preload(:people).limit(100).order(:id).to_a } }
    x.report('limit 1000 has_many') { 1.times { Item.preload(:events).limit(1000).order(:id).to_a } }
    x.report('limit 1000 has_many_through') { 1.times { Item.preload(:event_people).limit(1000).order(:id).to_a } }
    x.report('limit 1000 double has_many_through') { 1.times { Item.preload(:people).limit(1000).order(:id).to_a } }
  end
end

gistfile1.txt

$ ./run_bench.sh
---  Rails 3.2
3.2.21 - postgresql

limit 100 has_many                         0.030000   0.000000   0.030000 (  0.044157)
limit 100 has_many_through                 0.020000   0.010000   0.030000 (  0.027665)
limit 100 double has_many_through          0.040000   0.000000   0.040000 (  0.037460)
limit 1000 has_many                        0.050000   0.000000   0.050000 (  0.065530)
limit 1000 has_many_through                0.140000   0.000000   0.140000 (  0.152469)
limit 1000 double has_many_through         0.240000   0.000000   0.240000 (  0.267607)

---  Rails 4.1
4.1.8 - postgresql

limit 100 has_many                         0.020000   0.020000   0.040000 (  0.045980)
limit 100 has_many_through                 0.070000   0.020000   0.090000 (  0.086634)
limit 100 double has_many_through          0.100000   0.000000   0.100000 (  0.114789)
limit 1000 has_many                        0.060000   0.000000   0.060000 (  0.066844)
limit 1000 has_many_through                0.540000   0.020000   0.560000 (  0.573936)
limit 1000 double has_many_through         0.910000   0.070000   0.980000 (  0.997398)

---  Rails 4.2
4.2.0 - postgresql

limit 100 has_many                         0.040000   0.030000   0.070000 (  0.068367)
limit 100 has_many_through                 0.090000   0.020000   0.110000 (  0.112198)
limit 100 double has_many_through          0.120000   0.010000   0.130000 (  0.138451)
limit 1000 has_many                        0.070000   0.010000   0.080000 (  0.081651)
limit 1000 has_many_through                0.610000   0.020000   0.630000 (  0.644392)
limit 1000 double has_many_through         1.080000   0.080000   1.160000 (  1.180719)

---  Rails 4.2 edge
4.2.0 - postgresql

limit 100 has_many                         0.030000   0.010000   0.040000 (  0.042943)
limit 100 has_many_through                 0.060000   0.010000   0.070000 (  0.074245)
limit 100 double has_many_through          0.100000   0.010000   0.110000 (  0.109340)
limit 1000 has_many                        0.060000   0.000000   0.060000 (  0.072432)
limit 1000 has_many_through                0.520000   0.030000   0.550000 (  0.563653)
limit 1000 double has_many_through         0.900000   0.060000   0.960000 (  0.980529)