Postgres Cheat Sheets
The publisher of this site is a cheat sheet animal! While the documents might be a few years old, they still are relevant. But then again, most Postgres books are quite old, and frequently slammed in reviews for being either incomplete, out of date, or vague. Therefore, Postgres adopters largely are left to learn the technology on their own. From that perspective, these cheat sheets are extremely helpful at shortening the learning curve.