Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (q *Query) Hint(indexKey ...string) *Query {
	q.m.Lock()
	keyInfo, err := parseIndexKey(indexKey)
	q.op.options.Hint = keyInfo.key
	q.op.hasOptions = true
	q.m.Unlock()
	if err != nil {
		panic(err)
	}
	return q
}
func (q *Query) Hint(indexKey ...string) *Query {
	q.m.Lock()
	keyInfo, err := parseIndexKey(indexKey)
	q.op.options.Hint = keyInfo.key
	q.op.hasOptions = true
	q.m.Unlock()
	if err != nil {
		panic(err)
	}
	return q
}
// Hint will include an explicit "hint" in the query to force the server
// to use a specified index, potentially improving performance in some
// situations.  The provided parameters are the fields that compose the
// key of the index to be used.  For details on how the indexKey may be
// built, see the EnsureIndex method.
//
// For example:
//
//     query := collection.Find(bson.M{"firstname": "Joe", "lastname": "Winter"})
//     query.Hint("lastname", "firstname")
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Optimization
//     http://www.mongodb.org/display/DOCS/Query+Optimizer
//