I've done research against these database programs, and this is really really old news for anyone who has done testing. If you have a single machine, then Oracle is the best performing database, followed by Postgres. When you need more than 4 dedicated servers hosting a database, then mongo can handle about 180% of the volume that oracle can, and about 220% the volume of postgres, and about 110% the volume of Casandra.
As soon as you need more than one machine to host your database (which usually happens around 1000 active users on your website at any given time, depending on your application), consider switching off of an SQL database.