Things you didn't know about indexes
Let’s start with things you probably did know. 

 A database index is similar to the index you found at the back of your science textbooks in school. You want to find the pages that talk about Phosphorus? Head to the back and you’ll find an alphabetical list of topics alongside page numbers that reference them. 
 ...
 Oxygen .................. 42, 88, 103
 Periodic…