Blog Post: Speeding up ActiveRecord iterations on larger datasets

active_record_benchmark.rb

N = 1_000

Benchmark.bm do |x|
  x.report("all columns") do 
    N.times do
      User.find_by_sql("SELECT * FROM users LIMIT 1000;")
    end
  end

  x.report("one column") do 
    N.times do
      User.find_by_sql("SELECT email FROM users LIMIT 1000;")
    end
  end
end

       user     system      total        real
all columns 29.530000   2.280000  31.810000 ( 34.459085)
one column 19.110000   1.040000  20.150000 ( 21.719567)

looping_with_direct_query.rb

ActiveRecord::Base.connection.execute("SELECT email FROM users;").each do |record|
  email = record["email"]
  # ...
end

looping_with_select.rb

User.select(:email).find_each do |user|
  # ...
end

query_benchmark.rb

require 'benchmark'

N = 1_000
conn = ActiveRecord::Base.connection

Benchmark.bm do |x|
  # Include a random OFFSET in below queries so cached results are not returned
  
  x.report("all columns") do 
    N.times do
      conn.execute("SELECT * FROM users LIMIT 1000 OFFSET #{(1000*rand).round};")
    end
  end

  x.report("one column") do 
    N.times do
      conn.execute("SELECT email FROM users LIMIT 1000 OFFSET #{(1000*rand).round};")
    end
  end
end


       user     system      total        real
all columns  1.920000   2.510000   4.430000 ( 16.687199)
one column  0.370000   0.260000   0.630000 (  2.804193)