Godeps/_workspace/src/gopkg.in/mgo.v2/session.go
func (q *Query) Explain(result interface{}) error {
	q.m.Lock()
	clone := &Query{session: q.session, query: q.query}
	q.m.Unlock()
	clone.op.options.Explain = true
	clone.op.hasOptions = true
	if clone.op.limit > 0 {
		clone.op.limit = -q.op.limit
	}
	iter := clone.Iter()
	if iter.Next(result) {
		return nil
	}
	return iter.Close()
}
func (q *Query) Explain(result interface{}) error {
	q.m.Lock()
	clone := &Query{session: q.session, query: q.query}
	q.m.Unlock()
	clone.op.options.Explain = true
	clone.op.hasOptions = true
	if clone.op.limit > 0 {
		clone.op.limit = -q.op.limit
	}
	iter := clone.Iter()
	if iter.Next(result) {
		return nil
	}
	return iter.Close()
}
// Explain returns a number of details about how the MongoDB server would
// execute the requested query, such as the number of objects examined,
// the number of times the read lock was yielded to allow writes to go in,
// and so on.
//
// For example:
//
//     m := bson.M{}
//     err := collection.Find(bson.M{"filename": name}).Explain(m)
//     if err == nil {
//         fmt.Printf("Explain: %#v\n", m)
//     }
//
// Relevant documentation:
//
//     http://www.mongodb.org/display/DOCS/Optimization
//     http://www.mongodb.org/display/DOCS/Query+Optimizer
//