code: func (q *Query) Prefetch(p float64) *Query {
  	q.m.Lock()
  	q.prefetch = p
  	q.m.Unlock()
  	return q
  }
docs: // Prefetch sets the 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:
  //
  //     query.Batch(200).Prefetch(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-session basis as well, using the SetPrefetch method of Session.
  //
  // The default prefetch value is 0.25.