There's a hell of a lot of useful stuff to learn (practical and theoretical) from relational databases. NoSQL has some useful ideas too. However, advocating that someone wanting to be considered a "database administrator" only learn/focus on NoSQL - that's just poor advice.
Most DBAs will already know Oracle and MySQL and my point was that they would be much better advised learning a NoSQL database. Of course if you're a new DBA you should learn a relational database I just fail to see why on earth anybody would recommend PostgreSQL over Oracle/MySQL which are far, far, more popular.
That's what I was responding to. "looking to become a DBA" does not remotely imply someone who already "knows" Oracle and MySQL.
If you want a solid open-source database, especially one that has a future, learn Postgres.
So I guess if all of those clueless engineers at the world's most trafficked websites would have just listened to you they would never have picked MySQL. Have you ever thought of letting them know that they should switch over to PostgreSQL ?
If however you need to make a choice today for what database to use on a new project, then you should evaluate the choices instead of basing your decision on what might have been best for Facebook 8 years ago.
I'm not going to get into the technical differences here, because even if MySQL and Postgres were technically equivalent (and they aren't but I'll let you do your own research), I simply don't trust Oracle with the future of MySQL.
Speaking as a battle-scarred, proven old sage veteran of the DB wars? Because the opinions you state above don't really qualify you as that...
If you aren't interested in learning about MongoDB, MySQL, DB2, and stack of brittle punch cards, you're probably not cut out to be an IT professional in the first place.
And you're right. Scalability isn't everything hence the popularity of NoSQL which are far more developer friendly than current SQL databases. And in most companies it is the development teams that dictate technology choices.
Which means that all of the really hard scalability and management problems have already been solved every which way. And all available for free by companies you know and trust e.g. Facebook, Twitter.
If I were looking to become a DBA today I wouldn't bother with PostgreSQL at all. I would learn all of the NoSQL databases because that is where all of the interest is right now. Due to the increase use of agile in software development databases need to be flexible with the schema controlled by developers not DBAs.