Before PostgreSQL added BSON support this may have been true, but now it's pretty easy to build an SQL database that you can extend arbitrarily just like with M

Before PostgreSQL added BSON support this may have been true, but now it's pretty easy to build an SQL database that you can extend arbitrarily just like with Mongo. You probably shouldn't use that too much, not for anything remotely production, but it's there if you need that flexibility or an easy place to dump javascript objects. When you are just getting started, it's super easy to manipulate SQL data structures regardless.