distinct-active-record-bug.rb

gem 'rails', github: 'rails/rails'

require 'active_record'

`dropdb bug_1; createdb bug_1`

ActiveRecord::Base.establish_connection(
  adapter: 'postgresql',
  database: 'bug_1',
  host: 'localhost',
  username: 'postgres'
)

ActiveRecord::Schema.define do
  create_table :settings do |t|
    t.string :key
    t.string :value
  end
end

class Setting < ActiveRecord::Base
end

Setting.create!(key: "a", value: "1")
Setting.create!(key: "a", value: "2")
Setting.create!(key: "c", value: "3")

puts "Setting.select('DISTINCT key').size"
p Setting.select('DISTINCT key').size
puts "Setting.select('DISTINCT key').to_a.size"
p Setting.select('DISTINCT key').to_a.size
puts "Setting.select('DISTINCT ON (key) *').size"
p Setting.select('DISTINCT ON (key) *').size
puts "Setting.select('DISTINCT ON(key) *').to_a.size"
p Setting.select('DISTINCT ON(key) *').to_a.size