How to improve the execution time of a query in MongoDB?

To improve the execution time of a query, use index along with unique:true. Let us create a collection with documents −

> db.demo193.createIndex({"LastName":1},{unique:true});
{
   "createdCollectionAutomatically" : true,
   "numIndexesBefore" : 1,
   "numIndexesAfter" : 2,
   "ok" : 1
}
> db.demo193.insertOne({"FirstName":"John","LastName":"Doe"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e3ade1803d395bdc21346d1")
}
> db.demo193.insertOne({"FirstName":"John","LastName":"Smith"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e3ade1f03d395bdc21346d2")
}
> db.demo193.insertOne({"FirstName":"David","LastName":"Miller"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e3ade2803d395bdc21346d3")
}

Display all documents from a collection with the help of find() method −

> db.demo193.find();

This will produce the following output −

{ "_id" : ObjectId("5e3ade1803d395bdc21346d1"), "FirstName" : "John", "LastName" : "Doe" }
{ "_id" : ObjectId("5e3ade1f03d395bdc21346d2"), "FirstName" : "John", "LastName" : "Smith" }
{ "_id" : ObjectId("5e3ade2803d395bdc21346d3"), "FirstName" : "David", "LastName" : "Miller" }

Following is the query to find a value with quicker execution time −

> db.demo193.find({"LastName":"Smith"});

This will produce the following output −

{ "_id" : ObjectId("5e3ade1f03d395bdc21346d2"), "FirstName" : "John", "LastName" : "Smith" }