title: Changing MySQL query from username to email causes timeout
body: <p>If I run this code checking for <code>userid</code>, I get results in 1 second</p>
  <pre><code>    SELECT userid, username, email
      FROM db_main.users
      WHERE userid NOT IN 
      (
          SELECT userid 
          FROM db_backup.user 
      );
  </code></pre>
  <p>If I change it to <code>username</code>, I still get results in 1 second</p>
  <pre><code>    SELECT userid, username, email
      FROM db_main.users
      WHERE username NOT IN 
      (
          SELECT username 
          FROM db_backup.user 
      );
  </code></pre>
  <p>However, if I change it to <code>email</code>, then the query timeouts after 30 seconds.</p>
  <pre><code>    SELECT userid, username, email
      FROM db_main.users
      WHERE email NOT IN 
      (
          SELECT email 
          FROM db_backup.user 
      );
  </code></pre>
  <p>Any ideas why?
  Thanks!</p>