Index direction (ascending vs descending) in MongoDB - Hands-On Comparison

Create and Use Ascending and Descending Indexes in MongoDB

📖 Scenario: You are managing a small online bookstore database. You want to speed up searches on book titles and prices by creating indexes. You will create one index that sorts titles in ascending order and another index that sorts prices in descending order.

🎯 Goal: Build MongoDB indexes on the books collection: one index on title in ascending order and another index on price in descending order.

📋 What You'll Learn

Create a books collection with three documents containing title and price fields.

Create a variable called titleIndex that defines an ascending index on the title field.

Create a variable called priceIndex that defines a descending index on the price field.

Create the ascending index on title using db.books.createIndex() with titleIndex.

Create the descending index on price using db.books.createIndex() with priceIndex.

💡 Why This Matters

🌍 Real World

Indexes are used in real databases to speed up searches and sorting, just like an index in a book helps you find pages quickly.

💼 Career

Database administrators and developers create and manage indexes to optimize application performance and handle large data efficiently.

1

DATA SETUP: Create the books collection with sample documents

Insert three documents into the books collection with these exact entries: { title: "The Hobbit", price: 15 }, { title: "1984", price: 12 }, and { title: "Brave New World", price: 18 }.

MongoDB

// Insert the three book documents into the books collection
// Your code here

Hint

Use db.books.insertMany() with an array of objects for the books.

2

CONFIGURATION: Define index variables for ascending and descending order

Create a variable called titleIndex that sets title to 1 for ascending order, and a variable called priceIndex that sets price to -1 for descending order.

MongoDB

db.books.insertMany([
  { title: "The Hobbit", price: 15 },
  { title: "1984", price: 12 },
  { title: "Brave New World", price: 18 }
])

// Define titleIndex and priceIndex variables here
// Your code here

Hint

Use const to create the variables with the correct field and direction values.

3

CORE LOGIC: Create the ascending index on title

Use db.books.createIndex() with the variable titleIndex to create an ascending index on the title field.

MongoDB

db.books.insertMany([
  { title: "The Hobbit", price: 15 },
  { title: "1984", price: 12 },
  { title: "Brave New World", price: 18 }
])

const titleIndex = { title: 1 };
const priceIndex = { price: -1 };

// Create ascending index on title using titleIndex
// Your code here

Hint

Call db.books.createIndex() with titleIndex as the argument.

4

COMPLETION: Create the descending index on price

Use db.books.createIndex() with the variable priceIndex to create a descending index on the price field.

MongoDB

db.books.insertMany([
  { title: "The Hobbit", price: 15 },
  { title: "1984", price: 12 },
  { title: "Brave New World", price: 18 }
])

const titleIndex = { title: 1 };
const priceIndex = { price: -1 };

db.books.createIndex(titleIndex);

// Create descending index on price using priceIndex
// Your code here

Hint

Call db.books.createIndex() with priceIndex as the argument.

Practice

1. What does the index direction 1 mean in MongoDB indexes?

easy

A. Descending order

B. No order

C. Ascending order

D. Random order

Solution

1. Step 1: Understand index direction values

   In MongoDB, index direction 1 means ascending order, and -1 means descending order.

2. Step 2: Match the value to the meaning

   Since the question asks about 1, it corresponds to ascending order.

3. Final Answer:

   Ascending order -> Option C

4. Quick Check:

   Index direction 1 = Ascending order [OK]

Hint: 1 means ascending, -1 means descending [OK]

Common Mistakes:

• Confusing 1 with descending order
• Thinking 1 means no order
• Mixing up ascending and descending

2. Which of the following is the correct syntax to create a descending index on the field age in MongoDB?

easy

A. db.collection.createIndex({ age: 1 })

B. db.collection.createIndex({ age: -1 })

C. db.collection.createIndex({ age: 'desc' })

D. db.collection.createIndex({ age: 'descending' })

Solution

1. Step 1: Recall MongoDB index syntax for direction

   MongoDB uses 1 for ascending and -1 for descending in index creation.

2. Step 2: Identify the correct descending syntax

   The correct way to create a descending index on age is { age: -1 }.

3. Final Answer:

   db.collection.createIndex({ age: -1 }) -> Option B

4. Quick Check:

   Descending index uses -1 [OK]

Hint: Use -1 for descending index direction [OK]

Common Mistakes:

• Using string values like 'desc' instead of -1
• Using 1 which means ascending
• Incorrect object syntax

3. Given the index { name: 1, score: -1 }, which query will use this index efficiently?

medium

A. db.collection.find().sort({ name: 1, score: -1 })

B. db.collection.find().sort({ name: -1, score: 1 })

C. db.collection.find().sort({ score: -1, name: 1 })

D. db.collection.find().sort({ name: 1 })

Solution

1. Step 1: Understand compound index direction

   The index sorts name ascending and score descending.

2. Step 2: Match query sort order with index

   Only the query sorting { name: 1, score: -1 } matches the index directions exactly, so it uses the index efficiently.

3. Final Answer:

   db.collection.find().sort({ name: 1, score: -1 }) -> Option A

4. Quick Check:

   Sort matches index directions exactly [OK]

Hint: Sort order must match index directions exactly [OK]

Common Mistakes:

• Reversing sort directions
• Changing field order in sort
• Using partial sort that doesn't match index

4. You created an index with { date: 1, price: -1 } but your query sorts by { price: -1, date: 1 }. What is the problem?

medium

A. The index fields order does not match the sort order

B. The index direction values are invalid

C. MongoDB does not support mixed ascending and descending indexes

D. The query syntax is incorrect

Solution

1. Step 1: Check index field order

   The index is created with fields date then price.

2. Step 2: Compare query sort order

   The query sorts by price first, then date, which does not match the index field order.

3. Final Answer:

   The index fields order does not match the sort order -> Option A

4. Quick Check:

   Field order in sort must match index order [OK]

Hint: Index and sort field order must match exactly [OK]

Common Mistakes:

• Assuming direction values are invalid
• Thinking MongoDB disallows mixed directions
• Blaming query syntax instead of order

5. You want to optimize queries that sort by category ascending and then by rating descending. Which index should you create?

hard

A. db.collection.createIndex({ rating: -1, category: 1 })

B. db.collection.createIndex({ category: -1, rating: 1 })

C. db.collection.createIndex({ rating: 1, category: -1 })

D. db.collection.createIndex({ category: 1, rating: -1 })

Solution

1. Step 1: Identify query sort order

   The queries sort by category ascending and rating descending.

2. Step 2: Create matching compound index

   The index must have category with direction 1 and rating with direction -1 in that order.

3. Final Answer:

   db.collection.createIndex({ category: 1, rating: -1 }) -> Option D

4. Quick Check:

   Index matches query sort directions and order [OK]

Hint: Match index fields and directions to query sort exactly [OK]

Common Mistakes:

• Reversing field order in index
• Swapping ascending and descending directions
• Creating index on wrong fields