Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (s *Session) SetPrefetch(p float64) {
	s.m.Lock()
	s.queryConfig.prefetch = p
	s.m.Unlock()
}
func (s *Session) SetPrefetch(p float64) {
	s.m.Lock()
	s.queryConfig.prefetch = p
	s.m.Unlock()
}
// 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.