http://blog.darkhax.com/2010/05/14/find-queries-missing-indexes-in-your-rails-application

find_bad_queries.rb

require ENV_PATH

class String
  # http://www.devarticles.com/c/a/Development-Cycles/Tame-the-Beast-by-Matching-Similar-Strings/3/
  module Soundex
    Codes = {
      'b' => 1,
      'f' => 1,
      'p' => 1,
      'v' => 1,
      'c' => 2,
      'g' => 2,
      'j' => 2,
      'k' => 2,
      'q' => 2,
      's' => 2,
      'x' => 2,
      'z' => 2,
      'd' => 3,
      't' => 3,
      'l' => 4,
      'm' => 5,
      'n' => 5,
      'r' => 6
    }
  end

  def phonetic_code
    chars = self.split(//)
    initial = chars.shift.upcase
    chars.map { |c| Soundex::Codes[c.downcase] }.flatten.unshift(initial).to_s
  end

  def remove_color_codes
    self.gsub(/\x1B\[([0-9]{1,2}(;[0-9]{1,2})?)?[m|K]/, '')
  end
end

@codes = {}

File.open(ActiveRecord::Base.logger.instance_eval { @logdev }.filename, 'r') do |f|
  f.each_line do |line|
    if line.match(/ SELECT /)
      l = 'SELECT' + line.remove_color_codes.split('SELECT').last
      @codes[l.phonetic_code] = l
    end
  end
end

bad_queries = @codes.values.select do |query|
  ActiveRecord::Base.connection.select_all("EXPLAIN #{query}").any? do |result|
    result['key'].nil?
  end
end

puts bad_queries