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)

Just for kicks ;) I redid the above: ~two years later, on the same machine, with activerecord 5.0.0.1 and pg 0.19.0:

ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-linux]
Warming up --------------------------------------
           SQL query   353.000  i/100ms
       exception hit    62.811k i/100ms
      exception miss   297.570k i/100ms
Calculating -------------------------------------
           SQL query      2.921k (±32.9%) i/s -     13.061k in   5.052108s
       exception hit    831.142k (± 3.2%) i/s -      4.208M in   5.068312s
      exception miss     10.467M (± 3.2%) i/s -     52.372M in   5.008549s

Comparison:
      exception miss: 10466869.0 i/s
       exception hit:   831142.0 i/s - 12.59x slower
           SQL query:     2921.4 i/s - 3582.81x slower

ruby 2.4.0preview2 (2016-09-09 trunk 56129) [x86_64-linux]
Warming up --------------------------------------
           SQL query   262.000  i/100ms
       exception hit    62.278k i/100ms
      exception miss   344.852k i/100ms
Calculating -------------------------------------
           SQL query      3.306k (±24.1%) i/s -     15.458k in   5.080964s
       exception hit    871.144k (± 0.9%) i/s -      4.359M in   5.004697s
      exception miss     11.577M (± 1.1%) i/s -     57.935M in   5.005123s

Comparison:
      exception miss: 11576670.6 i/s
       exception hit:   871144.2 i/s - 13.29x  slower
           SQL query:     3306.0 i/s - 3501.78x  slower

Meanwhile @andrzejkrzywda duly pointed out that comparing exception handling time with writes (rather than – especially easily cacheable – reads) would be much more telling, so here’s the comparison with Whatever.create(text: 'meh') instead of Whatever.count:

ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-linux]
Warming up --------------------------------------
           SQL query    18.000  i/100ms
       exception hit    57.640k i/100ms
      exception miss   269.546k i/100ms
Calculating -------------------------------------
           SQL query    182.124  (±15.9%) i/s -    882.000  in   5.003193s
       exception hit    808.613k (± 4.8%) i/s -      4.035M in   5.004163s
      exception miss     10.129M (± 3.2%) i/s -     50.675M in   5.007747s

Comparison:
      exception miss: 10129279.5 i/s
       exception hit:   808613.1 i/s - 12.53x slower
           SQL query:      182.1 i/s - 55617.43x slower

ruby 2.4.0preview2 (2016-09-09 trunk 56129) [x86_64-linux]
Warming up --------------------------------------
           SQL query    20.000  i/100ms
       exception hit    58.768k i/100ms
      exception miss   344.645k i/100ms
Calculating -------------------------------------
           SQL query    186.218  (±20.4%) i/s -    880.000  in   5.005586s
       exception hit    818.088k (± 6.9%) i/s -      4.114M in   5.069060s
      exception miss     11.347M (± 0.6%) i/s -     56.866M in   5.011838s

Comparison:
      exception miss: 11346771.3 i/s
       exception hit:   818087.5 i/s - 13.87x  slower
           SQL query:      186.2 i/s - 60932.79x  slower

For me the take home is that when a (SQL) database is involved efforts to optimize the queries has a greater chance of improving app performance than futzing with flow control techniques. Not a surprise but it's nice to have some real numbers to point to. Thanks.