Simply said this book by Markus Winand should not be missing in any shelf of any database engineer. It does not explain basic knowledge about SQL, which you might learn from many others SQL-oriented books or from the Internet, but it gives a great background on the inner database implementations.
Why is knowing this background so crucial for database programmers? A general rule sais, if somebody programmes in some language, he should know basics of what is happening in two layers down. For example, a Python programmer should know at least basics of programming in C and should also know something about assembly language. Moving this theory to databases world, a programmer who works with SQL, should know about inner design of DBMS and should also know at least basics of C language. And the book from the title gives you that knowledge of DBMS.
SQL Performance Explained does not prioritize any DBMS, but tries to cover all the most famous DBMS out there -- Oracle DB, MySQL, SQL Server and (my favourite) PostgreSQL. A reader learns the magic behind database indexing implementation in the beginning, since it is the key for good performance in the first place. Later, we learn how created indexes are actually used in WHERE or ORDER BY clauses and how to help DBMS to actually use them.
Since the book is quite fresh, it covers even some newer features that are becoming a part of DBMS in the latest versions, like index-ony scans. As an icing on the cake, readers will find useful some very concrete examples. One example for all, Markus explains how a bit more modern way of pagination on the web pages using "later" instead of classic numbered pagination can make quite big performance difference in some cases.
Well, the not even 200 pages book is not the cheapest one, which is usual for such a book for quite narrow set of readers though. But it is definitely worth reading and looking time to time.
Good work, Markus, and thanks for that!