bench.rb

require 'benchmark'

keys = []
@uniq_keys = []

def add_key(key)
  @uniq_keys << key unless @uniq_keys.include?(key)
end

hasher1 = {}
50000.times do |x|
  srand(x)
  key = rand(10000).to_s

  keys << key
  add_key key
  hasher1[key] = x;
end

hasher2 = {}
50000.times do |x|
  srand(x);
  key = rand(10000).to_s

  keys << key
  add_key key
  hasher2[key] = x;
end

big = (Array(hasher1) | Array(hasher2)).map(&:first)
raise "NotEqual! #{big.count} - #{@uniq_keys.count}" unless big == @uniq_keys
raise "NotEqual!" unless keys.uniq == @uniq_keys

Benchmark.bmbm do |x|
  x.report{ (Array(hasher1) | Array(hasher2)).map(&:first) }
  x.report{ keys.uniq! }
  x.report{ @uniq_keys }
end

results.txt

Rehearsal ------------------------------------
   0.210000   0.000000   0.210000 (  0.213641)
   0.050000   0.000000   0.050000 (  0.054077)
   0.000000   0.000000   0.000000 (  0.000009)
--------------------------- total: 0.260000sec

       user     system      total        real
   0.200000   0.000000   0.200000 (  0.201144)
   0.010000   0.000000   0.010000 (  0.006215)
   0.000000   0.000000   0.000000 (  0.000011)

Pretty useless w/o also benching runtime of the setup, but interesting all the same.