ActiveRecord group_by performance. All the "USING BLOCK" tests are directly or indirectly using `Enumerable#group_by`, whereas the "USING SYMBOL" tests are using the new `ActiveRecord::FinderMethods#group_by`.

group_by_new.rb

require 'benchmark'

require File.expand_path('../../../load_paths', __FILE__)
require "active_record"

RECORDS = 20000


conn = { :adapter => 'sqlite3', :database => ':memory:' }

ActiveRecord::Base.establish_connection(conn)

class User < ActiveRecord::Base
  connection.create_table :users, :force => true do |t|
    t.string :name, :role
    t.timestamps
  end
end


puts 'Generating data...'

module ActiveRecord
  class Faker
    LOREM = %Q{Lorem ipsum dolor sit amet, consectetur adipiscing elit. Suspendisse non aliquet diam. Curabitur vel urna metus, quis malesuada elit.
     Integer consequat tincidunt felis. Etiam non erat dolor. Vivamus imperdiet nibh sit amet diam eleifend id posuere diam malesuada. Mauris at accumsan sem.
     Donec id lorem neque. Fusce erat lorem, ornare eu congue vitae, malesuada quis neque. Maecenas vel urna a velit pretium fermentum. Donec tortor enim,
     tempor venenatis egestas a, tempor sed ipsum. Ut arcu justo, faucibus non imperdiet ac, interdum at diam. Pellentesque ipsum enim, venenatis ut iaculis vitae,
     varius vitae sem. Sed rutrum quam ac elit euismod bibendum. Donec ultricies ultricies magna, at lacinia libero mollis aliquam. Sed ac arcu in tortor elementum
     tincidunt vel interdum sem. Curabitur eget erat arcu. Praesent eget eros leo. Nam magna enim, sollicitudin vehicula scelerisque in, vulputate ut libero.
     Praesent varius tincidunt commodo}.split

    def self.name
      LOREM.grep(/^\w*$/).sort_by { rand }.first(2).join ' '
    end
  end
end

# pre-compute the insert statements and fake data compilation,
# so the benchmarks below show the actual runtime for the execute
# method, minus the setup steps

today = Date.today
roles = %w(boss-man lowly-worker)

puts "Inserting #{RECORDS} users and exhibits..."
RECORDS.times do
  User.create(
    :created_at => today,
    :name       => ActiveRecord::Faker.name,
    :role       => roles.sample
  )
end


puts "SIZES USING SYMBOL:"
puts Benchmark.measure {
  groups = User.all.group_by(:role)
  groups.values.each do |users|
    users.size
  end
}

puts "SIZES USING BLOCK:"
puts Benchmark.measure {
  groups = User.all.group_by { |u| u.role }
  groups.values.each do |users|
    users.size
  end
}


puts "EACH USING SYMBOL:"
puts Benchmark.measure {
  groups = User.all.group_by(:role)
  groups.values.each do |users|
    users.each do |user|
    end
  end
}

puts "EACH USING BLOCK:"
puts Benchmark.measure {
  groups = User.all.group_by { |u| u.role }
  groups.values.each do |users|
    users.each do |user|
    end
  end
}

group_by_old.rb

require 'benchmark'

require File.expand_path('../../../load_paths', __FILE__)
require "active_record"

RECORDS = 20000


conn = { :adapter => 'sqlite3', :database => ':memory:' }

ActiveRecord::Base.establish_connection(conn)

class User < ActiveRecord::Base
  connection.create_table :users, :force => true do |t|
    t.string :name, :role
    t.timestamps
  end
end


puts 'Generating data...'

module ActiveRecord
  class Faker
    LOREM = %Q{Lorem ipsum dolor sit amet, consectetur adipiscing elit. Suspendisse non aliquet diam. Curabitur vel urna metus, quis malesuada elit.
     Integer consequat tincidunt felis. Etiam non erat dolor. Vivamus imperdiet nibh sit amet diam eleifend id posuere diam malesuada. Mauris at accumsan sem.
     Donec id lorem neque. Fusce erat lorem, ornare eu congue vitae, malesuada quis neque. Maecenas vel urna a velit pretium fermentum. Donec tortor enim,
     tempor venenatis egestas a, tempor sed ipsum. Ut arcu justo, faucibus non imperdiet ac, interdum at diam. Pellentesque ipsum enim, venenatis ut iaculis vitae,
     varius vitae sem. Sed rutrum quam ac elit euismod bibendum. Donec ultricies ultricies magna, at lacinia libero mollis aliquam. Sed ac arcu in tortor elementum
     tincidunt vel interdum sem. Curabitur eget erat arcu. Praesent eget eros leo. Nam magna enim, sollicitudin vehicula scelerisque in, vulputate ut libero.
     Praesent varius tincidunt commodo}.split

    def self.name
      LOREM.grep(/^\w*$/).sort_by { rand }.first(2).join ' '
    end
  end
end

# pre-compute the insert statements and fake data compilation,
# so the benchmarks below show the actual runtime for the execute
# method, minus the setup steps

today = Date.today
roles = %w(boss-man lowly-worker)

puts "Inserting #{RECORDS} users and exhibits..."
RECORDS.times do
  User.create(
    :created_at => today,
    :name       => ActiveRecord::Faker.name,
    :role       => roles.sample
  )
end


puts "SIZES USING BLOCK:"
puts Benchmark.measure {
  groups = User.all.group_by { |u| u.role }
  groups.values.each do |users|
    users.size
  end
}

puts "EACH USING BLOCK:"
puts Benchmark.measure {
  groups = User.all.group_by { |u| u.role }
  groups.values.each do |users|
    users.each do |user|
    end
  end
}

results.txt

$ git checkout master
$ ruby activerecord/examples/group_by_old.rb
Generating data...
Inserting 20000 users and exhibits...
SIZES USING BLOCK:
  0.700000   0.010000   0.710000 (  0.722346)
EACH USING BLOCK:
  0.680000   0.000000   0.680000 (  0.700059)
$ git checkout group_by-B
$ ruby activerecord/examples/group_by_new.rb
Generating data...
Inserting 20000 users and exhibits...
SIZES USING SYMBOL:
  0.030000   0.000000   0.030000 (  0.070316)
SIZES USING BLOCK:
  0.620000   0.020000   0.640000 (  0.660111)
EACH USING SYMBOL:
  0.620000   0.010000   0.630000 (  0.631824)
EACH USING BLOCK:
  0.690000   0.000000   0.690000 (  0.687753)