Postgres has some of the best documentation of any software product I’ve ever used. If someone wants to learn about SQL or databases, I always have to restrain myself from recommending that they just read the Postgres manual front to back. It’s comprehensive, it’s well written, it’s easy to navigate, and almost every explanation comes with several clear examples. It’s hard to overstate how valuable a property that is for a product as central to your architecture as your primary transactional database.