title: Faster way to find the last updated timestamp
body: <p>I expect this call</p>
  
  <pre><code>Model.maximum(:updated_at)
  </code></pre>
  
  <p>to be faster than this one</p>
  
  <pre><code>Model.order(:updated_at).last.updated_at
  </code></pre>
  
  <p>Is anyone able to confirm this assertion? and, if true, explain why?</p>