Ask HN: Resources on Postgres for a person who last did RDBMS some 15 years back

Hello HN, I last studied Databases (by which I mean SQL) a while back. Now, with new functional requirements such as High Availability, Scalability, Graph Databases etc being an integral part of modern day applications, how do you recommend I should proceed to get a complete grasp on Postgresql?

I wrote postgresql because it seems to be the most popular and powerful RDBMS around nowadays. I might be wrong.

Regards.