MySQL query planner fail.

wth_mysql.rb

#!/usr/bin/env ruby
#
# $ ./wth_mysql.rb
#                 user     system      total        real
# mysql       0.020000   0.010000   0.030000 (  0.617876)
# sqlite      0.000000   0.000000   0.000000 (  0.001234)
# postgresql  0.000000   0.000000   0.000000 (  0.002423)

srand 123456789
require 'active_record'
require 'benchmark'

DBCONFIG = {
  'sqlite' => {
    :adapter  => 'sqlite3',
    :database => 'test_planner.sqlite3'
  },
  'mysql' => {
    :adapter => 'mysql2',
    :database => 'test_planner'
  },
  'postgresql' => {
    :adapter => 'postgresql',
    :database => 'test_planner',
    :min_messages => 'warning'
  }
}

class Article < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :article
end

def setup_database(db)
  ActiveRecord::Base.establish_connection DBCONFIG[db]

  ActiveRecord::Base.silence do
    ActiveRecord::Migration.verbose = false

    ActiveRecord::Schema.define do

      create_table :articles, :force => true do |t|
        t.string   :title
        t.text     :body
        t.boolean  :published, :null => false, :default => false
        t.timestamps
      end

      create_table :comments, :force => true do |t|
        t.belongs_to :article
        t.string     :email
        t.text       :body
        t.timestamps
      end

      add_index :comments, :article_id

    end

    articles = (1..100).map do |id|
      Article.create!(:title => "Article Number #{id}", :body => "blah" * 1000, :published => !(id % 20).zero?)
    end

    20000.times do |id|
      article = articles.sample
      article.comments.create! :email => "number#{id}@example.com", :body => "Comment #{id} on Article #{article.id}" * 20
    end
  end

end

Benchmark.bm(10) do |x|
  %w( mysql sqlite postgresql ).each do |db|
    setup_database db
    x.report db do
      Comment.joins(:article).
              where(:articles => {:published => true}).
              order('comments.id desc').limit(3).all
    end
  end
end