I highly recommend the book "SQL Performance Explained" by Markus Winand: https://www.amazon.com/Performance-Explained-Everything-Deve...

I haven't read the book but it has 5 stars with 19 reviews on Amazon. https://www.amazon.com/Performance-Explained-Everything-Deve...

@MarkusWindand: Thank you for the Websites [1],[2],[3] and the book »SQL Performance Explained«[4].

[1] https://modern-sql.com/

[2] https://use-the-index-luke.com/

[3] https://db-engines.com/

[4] https://www.amazon.de/SQL-Performance-Explained-Everything-p...

If this tickles you fancy, you should check out this site http://use-the-index-luke.com or a book by the same author https://www.amazon.com/Performance-Explained-Everything-Deve...

He goes into great detail to explain how exactly SQL indices work and how to leverage this to write queries.