Benchmark single join vs 2 with pluck

join_query_benchmark.rb

require 'active_record'
require 'activerecord-import'
require 'mysql2'
require 'benchmark'

ActiveRecord::Base.establish_connection(
  adapter:  'mysql2',
  host:     'localhost',
  username: 'root',
  database: 'joiner'
)

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class Author < ApplicationRecord
  has_many :posts, dependent: :destroy
end

class Post < ApplicationRecord
  belongs_to :author
end

def migrate
  ActiveRecord::Schema.define do
    create_table(:authors) do |t|
      t.string :name
      t.integer :power_level
    end

    create_table(:posts) do |t|
      t.references :author, null: false, foreign_key: true
      t.string :title
      t.text :content
    end

    add_index :authors, :power_level
  end
end

def populate
  authors = []
  posts = []
  10_000.times do |i|
    authors << Author.new(id: i + 1, name: "Author #{i}", power_level: rand(3))
    100.times do |j|
      posts << Post.new(title: "Post #{i}-#{j}", author_id: i + 1)
    end
  end
  Author.import!(authors)
  Post.import!(posts)
end

migrate
populate

puts 'Populated!'

n = 5000
Benchmark.bm do |benchmark|
  benchmark.report("With join\t\t") do
    n.times do
      Post.joins(:author).where(authors: { power_level: rand(3) })
    end
  end

  benchmark.report("With nested select\t") do
    n.times do
      Post.where(author: Author.where(power_level: rand(3)))
    end
  end

  benchmark.report("With pluck\t\t") do
    n.times do
      author_ids = Author.where(power_level: rand(3)).pluck(:id)
      Post.where(author_id: author_ids)
    end
  end
end