Игорь, спасибо большое за ответ.
Еще раз перечитал раздел Modeling Your Data и нашел ответ на вопрос:
Parent-child joins can be a useful technique for managing relationships when index-time performance is more important than search-time performance, but it comes at a significant cost. Parent-child queries can be 5 to 10 times slower than the equivalent nested query!