title: What is a multiple key index?
body: <pre><code>add_index :microposts, [:user_id, :created_at]
  </code></pre>
  
  <p>I was going through Michael Hartl's railstutorial and noticed he's using something called a <em>multiple key index</em>. I know it means Active Record uses both keys at the same time, but I'm not really sure what the advantages and disadvantages of using multiple key indexes are.</p>
  
  <p>If anyone can give an answer I would greatly appreciate it.</p>