When not to index in MongoDB - Mini Project: Build & Apply

When Not to Index in MongoDB

📖 Scenario: You are managing a MongoDB database for a small online bookstore. You want to understand when it is better not to create indexes on certain fields to keep the database efficient.

🎯 Goal: Learn to create a collection with sample book data, add a configuration variable to decide when to index, write conditional logic to create an index only if the document count is above the threshold, and finalize the collection setup without unnecessary indexes.

📋 What You'll Learn

Create a MongoDB collection named books with specific documents

Add a variable minIndexCount to decide the minimum number of documents before indexing

Write conditional logic to create an index on genre only if the number of documents is above minIndexCount

Ensure the collection does not have an index on genre if the document count is low

💡 Why This Matters

🌍 Real World

In real applications, creating indexes on small collections can waste storage and slow down writes. This project shows how to avoid unnecessary indexes.

💼 Career

Database administrators and backend developers must know when to create indexes to optimize performance and resource use.

1

Create the books collection with sample data

Create a MongoDB collection called books and insert exactly these three documents: { title: "Book A", genre: "Fiction", pages: 200 }, { title: "Book B", genre: "Non-Fiction", pages: 150 }, and { title: "Book C", genre: "Fiction", pages: 300 }.

MongoDB

// Insert the three specified documents into the books collection

Need a hint?

Use insertMany to add multiple documents at once.

2

Add a variable minIndexCount to control indexing

Create a variable called minIndexCount and set it to 5. This variable will decide the minimum number of documents needed before creating an index.

MongoDB

db.books.insertMany([
  { title: "Book A", genre: "Fiction", pages: 200 },
  { title: "Book B", genre: "Non-Fiction", pages: 150 },
  { title: "Book C", genre: "Fiction", pages: 300 }
])

// Create minIndexCount variable

Need a hint?

Use const to declare the variable minIndexCount.

3

Write a conditional index creation logic

Write code that counts documents in books and creates an index on genre only if the count is greater than or equal to minIndexCount. Use db.books.countDocuments() and db.books.createIndex({ genre: 1 }).

MongoDB

db.books.insertMany([
  { title: "Book A", genre: "Fiction", pages: 200 },
  { title: "Book B", genre: "Non-Fiction", pages: 150 },
  { title: "Book C", genre: "Fiction", pages: 300 }
])

const minIndexCount = 5

// Add conditional index creation code here

Need a hint?

Use countDocuments() to get the number of documents, then an if statement to check if it meets minIndexCount.

4

Finalize collection without unnecessary indexes

Ensure the books collection does not have an index on genre if the document count is less than minIndexCount. Do not create the index manually.

MongoDB

db.books.insertMany([
  { title: "Book A", genre: "Fiction", pages: 200 },
  { title: "Book B", genre: "Non-Fiction", pages: 150 },
  { title: "Book C", genre: "Fiction", pages: 300 }
])

const minIndexCount = 5

const count = db.books.countDocuments()
if (count >= minIndexCount) {
  db.books.createIndex({ genre: 1 })
}

// Finalize collection setup without unnecessary indexes

Need a hint?

Do not create the index manually if the count is below the threshold.