See http://marcgg.com/blog/2014/10/14/mysql-try-all-indexes-combinations-script/

mysql_index_madness.rb

require 'benchmark'

ActiveRecord::Base.logger = nil
ActiveRecord::Schema.verbose = false
ActiveRecord::Migration.verbose = false

# PARAMS

sql = ""
attributes = []
table = ""
min_permutations = 1
max_permutations = 3
repeat_for_bench = 100

# SETTING ALL COMBINATIONS

possibilities = []

(min_permutations..max_permutations).each do |i|
  possibilities += attributes.repeated_permutation(i).to_a
end

possibilities.select! do |perm|
  perm.map{|v| perm.grep(v).size}.inject(&:+) == perm.size
end

# BENCHMARK

puts "We have #{possibilities.count} possibilities"

possibilities.each_with_index do |perm, i|
  index_name = "madness_test_#{i}"
  begin
    ActiveRecord::Migration.add_index(table, perm, name: index_name)
    r = Benchmark.measure {
      repeat_for_bench.times{ ActiveRecord::Base.connection.execute(sql) }
    }
    ActiveRecord::Migration.remove_index(table, name: index_name)
    puts [i, perm.inspect, r].join(",")
  rescue Exception => e
    puts [i, perm.inspect, "#{e.class} - #{e.message}"].join(",")
  end
end