MongoDB document store model is very appreciated by developpers. But it also has limitations where MySQL is simply better.
Using MongoDB in conjonction with MySQL is becoming a very comon architectural pattern.
Comparing MongoDB, MySQL, and TokuMX Data Layout 2013-07-22 Zardosht Kasheff
pt-query-digest for MongoDB profiler logs 2013-07-23 hingo
Schema Design in MongoDB vs Schema Design in MySQL 2013-08-01 S. Combaudon
TokuMX Fractal Tree(R) indexes, what are they? 2013-07-02 Zardosht Kasheff