Question

I have a table with about 500,000 users in it. When I use my Rails 3.0 console and type

User.first

I get the result immediately. But when I type

User.first(1)

it takes several seconds. What could be causing this?

Note: the same thing happens between

User.last

and

User.last(1)

Answer 1

I turned on log displaying in Erb (Rails 3.0.7) and yield following

1.8.7 :004 > User.first
  User Load (4.3ms)  SELECT `users`.* FROM `users` LIMIT 1
 => #<User id: 1, email: "[e-mail]">
1.8.7 :005 > User.first(1)
  User Load (0.4ms)  SELECT `users`.* FROM `users`
 => #<User id: 1, email: "[e-mail]">
1.8.7 :008 > User.first(2)
  User Load (0.4ms)  SELECT `users`.* FROM `users`
 => [#<User id: 1, email: "[e-mail]", #<User id: 2, email: "[e-mail]">]

So we can see that when you call first without arguments it works as expected, but when we call first with a number as argument it loads all users for DB and then leave only number elements in users array.

In recent Rails code of first looks like this

def first(*args)
  if args.any?
    if args.first.kind_of?(Integer) || (loaded? && !args.first.kind_of?(Hash))
      limit(*args).to_a
    else
      apply_finder_options(args.first).first
    end
  else
    find_first
  end
end

So it is quite ok. But in older version like Rails 3.0.7 it is:

def first(*args)
  if args.any?
    if args.first.kind_of?(Integer) || (loaded? && !args.first.kind_of?(Hash))
      to_a.first(*args)
    else
      apply_finder_options(args.first).first
    end
  else
    find_first
  end
end

So as you can see if first argument is Integer ActiveRecord load all data, convert it into array and call Array’s first method, which has following behaviour: http://www.ruby-doc.org/core-1.9.3/Array.html#method-i-first