> A deployment that changes only the code is simpler than one that changes the DB.
In your experience, perhaps, but that's reversed in many other places.
As for everything else your point is only accurate if you assume that migrations are done by hand. If use a migration library it's impossible to forget to apply a migration to a database so there's no problem working with copies or even forks of databases.
> And if it was so safer and easy to do it PostgreSQL wouldn't have added the json field.
You're implying causation incorrectly: hstore and JSON are useful for cases where you explicitly do not want schema enforcement or can't afford the performance impact of normalization. This is not saying that migrations are hard, merely that not all problems have the same best solution.
