ordering.rb

require 'benchmark/ips'

module TestMethods
  def order_by_ids(ids)
    order("idx(ARRAY#{ids}, id)")
  end


  def order_by_ids_string(ids)
    o = Post.send(:sanitize_sql_array,
      ["position(id::text in ?)", ids.join(',')]
    )
    order(o)
  end

  def order_by_ids_string_fast(ids)
    order("position(id::text in '#{ids.join(',')}')")
  end

  def to_ordered_ary(ids)
    ids.map!(&:to_i)
    # self.select_values += [:id]
    to_a.index_by(&:id).values_at(*ids)
  end
end

ActiveRecord::Relation.send(:include, TestMethods)

ids = [80, 91, 87, 90, 86, 93, 92, 85, 84, 81, 79]
scp = Post.where(id: ids).select(:id)

Benchmark.ips do |x|
  x.report('intarray')  { scp.order_by_ids(ids).to_a }
  x.report('position')  { scp.order_by_ids_string(ids).to_a }
  x.report('position2') { scp.order_by_ids_string_fast(ids).to_a }
  x.report('ruby')      { scp.to_ordered_ary(ids) }
  x.compare!
end

Comparison:
                ruby:    96370.1 i/s
           position2:      725.5 i/s - 132.83x slower
            intarray:      708.7 i/s - 135.99x slower
            position:      690.4 i/s - 139.58x slower