title: pt-query-digest filter queries with no key or bad key
body: <p>How to filter all queries with either no key or a "bad" key?</p>
  
  <pre><code>pt-query-digest /var/lib/mysql/mysql-slow.log --filter '($event-&gt;{No_index_used} eq "Yes" || $event-&gt;{No_good_index_used} eq "Yes")'
  </code></pre>
  
  <p>This syntax returns evaluation error</p>