I purchased this book principally for learning SQL tunning/Performance for oracle database however the book covers several widely used DBMS such as oracle, PostgreSQL, mysql, sql server, MySQL, . . .
The domain is quite vast & as a beginner, if you just check the online oracle documentation, you can hardly find your way. This book opens the door. You learn the basic concepts and terminology (what is a BTree index, how it is read and how it can impact performance). There are many interesting examples, each of them, provided with execution plans.
At the end of the book there is a summary of how to read an execution plan (dedicated section for each DBMS) The author uses a simple language and in general it seems that the book addresses principally those who almost don't know much about the performance problems. Yet I believe non beginners can also benefit from the interesting scenarios described in the book for investigating and tackling performance problems (very good & classic examples)
The accent has been put on indexes. You will learn different type of indexes, how to improve them, how a database read an index, doing different type of index/table scans, how to investigate the cause of a slow query, etc.
Personally I liked the book and I recommend it. I would've preferred to see more details in the book about the meaning of each section of an execution plan. Also it would be more helpful if the author gives more details about hash joins and hash group by.