Sparse Index in mongodb

sparseIndex.txt

db.collection_name.insert([
  {_id: 1, name: "John Doe", age: 25},
  {_id: 2, name: "Foo Bar", age: 35},
  {_id: 3, name: "Hello World"}
]);

// create sparse index
db.scores.createIndex( { age: 1 } , { sparse: true } );

// count the sparse Indexes Field will output 2
db.scores.find().hint("index_name").count();

// create without sparse index
db.scores.createIndex( { age: 1 } );

// count the non-sparse Indexes Field will output 3
db.scores.find().hint("index_name").count();