I was conflicted when I read "3. Most selective columns to the left":

http://www.unofficialmysqlguide.com/composite-indexes.html#d...

Because I had previously read that it was a myth:

https://use-the-index-luke.com/sql/myth-directory/most-selec...

Both this guide and Use The Index Luke seem to be good resources. Later I realized that maybe the guide had listed this optimization tips in order of importance. After all "1. Leftmost rule" and "2. Ranges to the right" are the ones that affect the most the usability of the index. Then the seemingly opposite viewpoints converge.