Query patterns that cause collection scans in MongoDB - Mini Project: Build & Apply

Query Patterns That Cause Collection Scans in MongoDB

📖 Scenario: You are working with a MongoDB database for a small online bookstore. The database has a collection called books that stores information about each book, including its title, author, year of publication, and genre.Sometimes, queries run slowly because MongoDB has to scan the entire collection instead of using an index. This happens when queries use patterns that do not match any index.

🎯 Goal: Learn to write queries that cause collection scans in MongoDB by using patterns that do not use indexes. This will help you understand why some queries are slow and how to identify them.

📋 What You'll Learn

Create a books collection with specific documents.

Add an index on the author field.

Write a query that causes a collection scan by searching on a non-indexed field.

Write a query that causes a collection scan by using a regex pattern that cannot use the index.

💡 Why This Matters

🌍 Real World

Understanding which queries cause collection scans helps developers optimize database performance by creating appropriate indexes.

💼 Career

Database administrators and backend developers must know how to identify slow queries caused by collection scans to improve application responsiveness.

1

Create the books collection with sample documents

Create a variable called books and assign it an array of three documents with these exact fields and values: { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, genre: "Fantasy" }, { title: "1984", author: "George Orwell", year: 1949, genre: "Dystopian" }, and { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, genre: "Fiction" }. Then insert the documents into the books collection using db.books.insertMany(books).

MongoDB

const books = [
  // Your code here
];

Need a hint?

Use an array with three objects exactly as shown, then db.books.insertMany(books);

2

Create an index on the author field

Write a line of code that creates an index on the author field of the books collection using db.books.createIndex({ author: 1 }).

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, genre: "Fantasy" },
  { title: "1984", author: "George Orwell", year: 1949, genre: "Dystopian" },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, genre: "Fiction" }
];

db.books.insertMany(books);

// Your code here

Need a hint?

Use db.books.createIndex with { author: 1 } to create the index.

3

Write a query that causes a collection scan by searching on genre

Write a query using db.books.find() to find all books where genre is exactly "Fantasy". This query will cause a collection scan because genre is not indexed.

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, genre: "Fantasy" },
  { title: "1984", author: "George Orwell", year: 1949, genre: "Dystopian" },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, genre: "Fiction" }
];

db.books.insertMany(books);

db.books.createIndex({ author: 1 });

// Your code here

Need a hint?

Use db.books.find with a filter on genre equal to "Fantasy".

4

Write a query that causes a collection scan using a regex on title

Write a query using db.books.find() to find all books where the title starts with the letter "T" using a regex: { title: /^T/ }. This query will cause a collection scan because title is not indexed and the regex cannot use an index.

MongoDB

const books = [
  { title: "The Hobbit", author: "J.R.R. Tolkien", year: 1937, genre: "Fantasy" },
  { title: "1984", author: "George Orwell", year: 1949, genre: "Dystopian" },
  { title: "To Kill a Mockingbird", author: "Harper Lee", year: 1960, genre: "Fiction" }
];

db.books.insertMany(books);

db.books.createIndex({ author: 1 });

db.books.find({ genre: "Fantasy" });

// Your code here

Need a hint?

Use db.books.find with a regex filter on title starting with T.