cost of using exceptions for control flow compared to one SQL statement (ruby 2.1.4, rails 4.1.7, sqlite) for rails-refactoring.com . Development mode executed in rails console.

bench.rb

require 'benchmark'

ActiveRecord::Base.logger = nil

Benchmark.bmbm do |bench|
  bench.report("SQL query") do
    1000.times { Whatever.count }
  end

  bench.report("exception hit") do
    1000.times do
      begin
        raise StandardError.new
      rescue
      end
    end
  end

  bench.report("exception miss") do
    1000.times do
      begin
        raise StandardError.new if false
      rescue
      end
    end
  end
end

output.txt

Rehearsal --------------------------------------------------
SQL query        0.180000   0.010000   0.190000 (  0.199253)
exception hit    0.050000   0.000000   0.050000 (  0.050654)
exception miss   0.000000   0.000000   0.000000 (  0.000050)
----------------------------------------- total: 0.240000sec

                     user     system      total        real
SQL query        0.180000   0.010000   0.190000 (  0.179669)
exception hit    0.050000   0.000000   0.050000 (  0.048447)
exception miss   0.000000   0.000000   0.000000 (  0.000054)