How to efficiently run complex queries on MongoDB unindexed fields?

Create an index to efficiently run complex queries. Let us first create a collection with documents −

> db.demo400.insertOne({SubjectName:"Java Spring"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e610720fac4d418a0178572")
}
> db.demo400.insertOne({SubjectName:"Spring Hibernate"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e61072dfac4d418a0178573")
}
> db.demo400.insertOne({SubjectName:"Java Hibernate"});
{
   "acknowledged" : true,
   "insertedId" : ObjectId("5e610736fac4d418a0178574")
}
> db.demo400.createIndex({SubjectName:"text"});
{
   "createdCollectionAutomatically" : false,
   "numIndexesBefore" : 1,
   "numIndexesAfter" : 2,
   "ok" : 1
}

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

> db.demo400.find();

This will produce the following output −

{ "_id" : ObjectId("5e610720fac4d418a0178572"), "SubjectName" : "Java Spring" }
{ "_id" : ObjectId("5e61072dfac4d418a0178573"), "SubjectName" : "Spring Hibernate" }
{ "_id" : ObjectId("5e610736fac4d418a0178574"), "SubjectName" : "Java Hibernate" }

Following is the query to efficiently perform complex queries on unindexed fields −

> db.demo400.find({ $text: { $search: "Spring" } } )

This will produce the following output −

{ "_id" : ObjectId("5e61072dfac4d418a0178573"), "SubjectName" : "Spring Hibernate" }
{ "_id" : ObjectId("5e610720fac4d418a0178572"), "SubjectName" : "Java Spring" }