title: Changes in query behaviour
body: <p>I have some queries that run every day for several month with no problem. I didn't change anything in the queries for a long while.
  In the past few days some of them fail. Error message says something regarding some fields: "Field 'myfield' not found.". these queries usually involve some sub-queries and window functions.
  Example for the BQ guys: </p>
  
  <ul>
  <li>On 2015-08-03 Job ID: job_EUWyK5DIFSxJxGAEC4En2Q_hNO8 run successfully</li>
  <li>on the following days, same query, failed. Job IDs: (job_A9KYJLbQJQvHjh1g7Fc0Abd2qsc , job__15ff66aYseR-YjYnPqWmSJ30N8)</li>
  </ul>
  
  <p>In addition, for some other queries running times extended from minutes to hours and sometime return "timeout".</p>
  
  <p>My questions:</p>
  
  <ol>
  <li>Was something changed in the BQ engine?</li>
  <li>What should I do to make my queries run again?</li>
  </ol>
  
  <p>Thanks</p>