gistfile1.rb

#Для 1100 записів
[~/Projects/test2] ./script/performance/benchmarker 10 "Record.test_select" "Record.test_select2" "Record.test_select3" "Record.test_select4"

            user     system      total        real
#1      0.000000   0.000000   0.000000 (  0.112709)
#2      0.110000   0.000000   0.110000 (  0.168085)
#3      0.000000   0.000000   0.000000 (  0.068406)
#4      4.620000   0.030000   4.650000 (  6.498799)
[~/Projects/test2] ./script/performance/benchmarker 100 "Record.test_select" "Record.test_select2" "Record.test_select3" "Record.test_select4"

            user     system      total        real
#1      0.050000   0.010000   0.060000 (  0.935885)
#2      1.320000   0.010000   1.330000 (  2.005846)
#3      0.090000   0.010000   0.100000 (  0.459577)
#4     45.540000   0.170000  45.710000 ( 66.452645)

#Для 11100 записів
[~/Projects/test2] ./script/performance/benchmarker 10 "Record.test_select" "Record.test_select2" "Record.test_select3" "Record.test_select5"

            user     system      total        real
#1      0.000000   0.000000   0.000000 (  0.928129)
#2      1.280000   0.010000   1.290000 (  1.908505)
#3      0.000000   0.010000   0.010000 (  0.186427)
#4      0.020000   0.000000   0.020000 (  0.020361)
[~/Projects/test2] ./script/performance/benchmarker 100 "Record.test_select" "Record.test_select2" "Record.test_select3" "Record.test_select5"

            user     system      total        real
#1      0.050000   0.000000   0.050000 (  9.129972)
#2     13.030000   0.060000  13.090000 ( 19.909641)
#3      0.110000   0.000000   0.110000 (  2.104700)
#4      0.070000   0.010000   0.080000 (  0.230101)
[~/Projects/test2] 

record.rb

class Record < ActiveRecord::Base
 def self.test_select
   Record.first(:order => 'RAND() * records.weight DESC')
 end

 def self.test_select2
   records = Record.find(:all, :select => 'id', :order => 'records.weight DESC')
   Record.find(records[(rand()*records.count).to_i])
 end 

 def self.test_select3(weight = 5)
   Record.first(:conditions => [ "id >= ?", (rand()*Record.last.id).to_i], :order => 'records.weight DESC')
 end

 def self.test_select4
  records = Record.all(:select => 'id, weight')
  records_arr = []
  records.each do |r|
    r.weight.times{records_arr << r.id}
  end
  Record.find(records_arr[rand()*records_arr.size])
 end

 def self.test_select5
   Record.find((rand*Record.count).to_i, :order => 'records.weight DESC')
 end
end