Index direction (ascending vs descending) in MongoDB - Practice Questions

Challenge - 5 Problems

❓ query_result

intermediate

Effect of ascending index on query sort

Given a MongoDB collection products with an ascending index on price, what will be the order of price values returned by this query?

db.products.find().sort({price: 1})

A Documents sorted by price from lowest to highest

B Documents sorted by price from highest to lowest

C Documents returned in insertion order

D Documents sorted randomly

💡 Hint

Ascending index means sorting from smallest to largest.

An ascending index on price stores values from lowest to highest. Sorting with {price: 1} uses this index to return documents in ascending order.

❓ query_result

intermediate

Effect of descending index on query sort

Given a MongoDB collection orders with a descending index on orderDate, what will be the order of orderDate values returned by this query?

db.orders.find().sort({orderDate: -1})

A Documents sorted by orderDate ascending

B Documents sorted by orderDate from oldest to newest

C Documents returned in random order

D Documents sorted by orderDate from newest to oldest

💡 Hint

Descending index means sorting from largest to smallest.

A descending index on orderDate stores values from newest to oldest. Sorting with {orderDate: -1} uses this index to return documents in descending order.

🧠 Conceptual

advanced

Index direction impact on compound index queries

Consider a compound index on {category: 1, price: -1} in a MongoDB collection. Which query will benefit most from this index?

A db.items.find().sort({category: -1, price: 1})

B db.items.find({category: 'books'}).sort({category: 1, price: -1})

C db.items.find({price: {$gt: 10}}).sort({price: -1})

D db.items.find({category: 'books'}).sort({price: 1})

💡 Hint

The query must match the index fields and directions to use the index efficiently.

The compound index {category: 1, price: -1} supports queries filtering by category and sorting by price descending. Option B matches both fields and directions.

📝 Syntax

advanced

Correct syntax for creating descending index

Which of the following commands correctly creates a descending index on the score field in MongoDB?

A db.scores.createIndex({score: 0})

B db.scores.createIndex({score: 'desc'})

C db.scores.createIndex({score: -1})

D db.scores.createIndex({score: 'descending'})

💡 Hint

Index direction is specified by 1 for ascending or -1 for descending.

The correct syntax uses -1 to specify descending order. Other strings or zero are invalid.

❓ optimization

expert

Choosing index direction for frequent queries

You have a MongoDB collection events with frequent queries sorting by timestamp ascending and some queries sorting by timestamp descending. Which index direction should you choose to optimize overall performance?

A Create an ascending index on timestamp to optimize ascending sorts; descending sorts will scan backward

B Create a descending index on timestamp to optimize descending sorts; ascending sorts will scan backward

C Create two separate indexes: one ascending and one descending on timestamp

D Create a hashed index on timestamp for both ascending and descending sorts

💡 Hint

MongoDB can scan indexes backward to support opposite sort direction.

Creating an ascending index allows efficient ascending sorts and MongoDB can scan it backward for descending sorts, avoiding extra index creation.

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