title: rails - created_at when user for ordering, Should you add an Index to the table?
body: <p>Hello for one of my models photos I have:</p>
  
  <pre><code>default_scope :order =&gt; 'photos.created_at DESC, photos.version DESC'
  </code></pre>
  
  <p>Given that I'm ordering by CREATED_AT and Version... Should I have a DB index on CREATED_AT?</p>
  
  <p>Thanks</p>