Lots of mistakes (or at least rare opinions going against the crowd) here.
Here's a better general performance tuning handbook - https://use-the-index-luke.com/
Here's a better general performance tuning handbook - https://use-the-index-luke.com/
Use-the-index-luke is an altogether deeper, more technical article aimed at data engineers and going into the details and differences between databases.
I disagree. Professional developers should know their database of choice inside and out, and use-the-index-luke helps with that. You can skip the details about databases that aren't relevant to you.