database_bench.rb

require 'benchmark'

task :db_bench => :environment do
  Benchmark.bm do |b|
    b.report { 1.upto(10000) { User.first } }
    b.report { 1.upto(10000) { User.select([:email, :default_edition_id]).first } }

    b.report { 1.upto(10000) { User.first.created_at } }
    b.report { 1.upto(10000) { User.select([:created_at, :default_edition_id]).first.created_at } }

    b.report { 1.upto(10000) { ActiveRecord::Base.connection.execute("SELECT * FROM users LIMIT 1") } }
    b.report { 1.upto(10000) { ActiveRecord::Base.connection.execute("SELECT email FROM users LIMIT 1") } }
  end
end


   #     user     system      total        real
   # 2.980000   0.220000   3.200000 (  3.837946)
   # 2.410000   0.190000   2.600000 (  3.037000)
   # 3.070000   0.210000   3.280000 (  3.913174)
   # 2.620000   0.200000   2.820000 (  3.260302)
   # 0.810000   0.210000   1.020000 (  1.573247)
   # 0.740000   0.190000   0.930000 (  1.306029)

So 80μs saved per query. How many rows/columns?