But what I can say is that if you are considering Mongo plus another database like MySQL, then in all likelihood you shouldn’t do it. The benefits of being schemaless are negated by the pain you will feel sorting out: Logging. Monitoring. Slow query optimization. init scripts. Graphing. Replication. Sharding strategy. Rebalancing strategy. Backups. Restoration. Probably like 50 other things Allspaw knows about that we developers don’t have to care about.
(Full Story: http://mcfunley.com/why-mongodb-never-worked-out-at-etsy )