Add indexes on a collection's columns in mongodb

add_index.js

// Let the collection's name is 'students' & we want to speed up the query's performance on the 'students' collection when we find documents in the collection by 'student_id'. 
// In order to achieve this, we'll add an index for the 'student_id' column where we want to index it by ascending order.

db.students.ensureIndex({'student_id': 1}) 

// Another case is when we want to speed up query which takes both 'student_id' & 'class' columns where the class would be descending order.
db.students.ensureIndex({'student_id':1, 'class':-1})