I enjoyed this book a lot and recommend it to everyone who uses PostgreSQL or MySQL. MySQL users should benefit from understanding PostgreSQL. Beyond that, I learned a lot from this book that I can apply directly to MySQL. In particular, the book begins with a few chapters on hardware performance, benchmarking, and configuration. This material is database-agnostic and very well done. There is about 70 pages of it — it goes into a lot of details. It is more detailed than the similar material in my own book High Performance MySQL.
The rest of the book is much more focused on PostgreSQL. There are chapters on memory use, server configuration, maintenance (with a good survey of how PostgreSQL handles things like MVCC), benchmarking, indexing, query optimization, statistics, monitoring and trending, pooling, caching, replication, partitioning, proxies, and finally an extensive laundry list of common problems and how to solve them.
It was a pleasure to read — the quality and clarity of the writing is very good. Greg is an excellent writer and obviously put a lot of work into this book.