title: pg8000 and cursor.fetchall() failing to return records if the number of records is moderate
body: <p>I'm using the adaptor <code>pg8000</code> to read in records in my db with the following code:</p>
  
  <pre><code>cursor = conn.cursor()
  results = cursor.execute("SELECT * from data_" + country + " WHERE date &gt;= %s AND date &lt;= %s AND time &gt;= %s AND time &lt;= %s", (datetime.date(fromdate[0], fromdate[1], fromdate[2]), datetime.date(todate[0], todate[1], todate[2]),datetime.time(fromtime[0],fromtime[1]), datetime.time(totime[0],totime[1])))
  results = cursor.fetchall()
  </code></pre>
  
  <p>The problem emerges when I select a date range that brings in say 100 records. Its not a huge number of records but it is enough to cause the following issue and I cannot see where the issue might come from - as it seems to be dependent on the number of records brought back. For example: <code>results = cursor.fetchall()</code> appears to work just fine and return one result.</p>
  
  <p>The error message I get is:</p>
  
  <pre><code>File "/mnt/opt/Centos5.8/python-2.7.8/lib/python2.7/site-packages/pg8000/core.py", line 1650, in handle_messages
      raise error
  pg8000.errors.ProgrammingError: ('ERROR', '34000', 'portal "pg8000_portal_0" does not exist')
  </code></pre>
  
  <p>Obviously I cannot find a way of fixing this despite exploring.</p>
  
  <p>When using <code>fetchmany()</code>, here are the results:</p>
  
  <p><code>results = cursor.fetchmany(100)</code> <strong>WORKS</strong> - limited to 100</p>
  
  <p><code>results = cursor.fetchmany(101)</code> <strong>FAILS</strong> - same error as above</p>