datastore/query.go
func (q *Query) BatchSize(size int) *Query {
	q = q.clone()
	if size <= 0 || size > math.MaxInt32 {
		q.err = errors.New("datastore: query batch size overflow")
		return q
	}
	q.count = int32(size)
	return q
}
func (q *Query) BatchSize(size int) *Query {
	q = q.clone()
	if size <= 0 || size > math.MaxInt32 {
		q.err = errors.New("datastore: query batch size overflow")
		return q
	}
	q.count = int32(size)
	return q
}
// BatchSize returns a derivative query to fetch the supplied number of results
// at once. This value should be greater than zero, and equal to or less than
// the Limit.