I honestly think that for live operations an SQL database is more trouble than it's worth - sooner or later you need more control than it gives you, so you're better off using a datastore that gives you lower-level access to construct and use your own indices explicitly. SQL makes sense for reporting-type use cases where you don't know exactly what queries and aggregations you'll be doing ahead of time (but have a rough idea of which columns you might need to index on), but that's all.