code: func (s *Session) SetPrefetch(p float64) {
  	s.m.Lock()
  	s.queryConfig.prefetch = p
  	s.m.Unlock()
  }
docs: // SetPrefetch sets the default point at which the next batch of results will be
  // requested.  When there are p*batch_size remaining documents cached in an
  // Iter, the next batch will be requested in background. For instance, when
  // using this:
  //
  //     session.SetBatch(200)
  //     session.SetPrefetch(0.25)
  //
  // and there are only 50 documents cached in the Iter to be processed, the
  // next batch of 200 will be requested. It's possible to change this setting on
  // a per-query basis as well, using the Prefetch method of Query.
  //
  // The default prefetch value is 0.25.