title: mysql workbench takes too long time to apply a simple query
body: <p>this is my query,</p>
  
  <pre><code>    SELECT 
      COUNT(a.ref) AS refoct2015,
      COUNT(b.ref) AS refspet2015,
      COUNT(c.ref) AS refoct2014,
      port_ID
  FROM
      report AS a
          JOIN
      report_sept2015 AS b USING (port_ID)
          JOIN
      report_oct2014 AS c USING (port_ID)
  GROUP BY port_ID , a.booking_date , b.booking_date , c.booking_date
  </code></pre>
  
  <p>but sql takes more than 15 min to give me a result when it doesn't stop working.
  I restart it many times, changed the reading time (Edit>Preference>etc..) but nothing still very slow</p>