Index intersection behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Index Intersection Query Result

Given a MongoDB collection products with indexes on category and price, what documents will be returned by this query?

{ category: "electronics", price: { $lt: 100 } }

Assume documents:

[{_id:1, category:"electronics", price: 90}, {_id:2, category:"electronics", price: 150}, {_id:3, category:"furniture", price: 80}]

MongoDB

db.products.find({ category: "electronics", price: { $lt: 100 } })

A [{_id:1, category:"electronics", price: 90}]

B [{_id:1, category:"electronics", price: 90}, {_id:2, category:"electronics", price: 150}]

C [{_id:3, category:"furniture", price: 80}]

D []

💡 Hint

Think about which documents satisfy both conditions.

The query filters documents where category is "electronics" AND price is less than 100. Only the first document meets both.

🧠 Conceptual

intermediate

Understanding Index Intersection

What does MongoDB's index intersection feature do?

A Combines multiple single-field indexes to satisfy a query with multiple conditions.

B Removes unused indexes from the database.

C Creates a new compound index automatically for every query.

D Merges documents from different collections.

💡 Hint

Think about how MongoDB uses indexes to speed up queries with multiple filters.

Index intersection allows MongoDB to use multiple single-field indexes together to answer queries with multiple conditions efficiently.

📝 Syntax

advanced

Correct Index Creation Syntax

Which option correctly creates two single-field indexes on category and price in MongoDB?

A db.products.createIndex(["category", "price"])

B db.products.createIndex({ category: "asc", price: "asc" })

C db.products.createIndex({ category: 1 }); db.products.createIndex({ price: 1 })

D db.products.createIndex({ category: 1, price: 1 })

💡 Hint

Remember how to create single-field indexes separately.

Option C creates two separate single-field indexes, one on category and one on price. Option C creates a compound index. Options B and C use invalid syntax.

❓ optimization

advanced

Improving Query Performance with Index Intersection

You have a query filtering on status and date. You currently have single-field indexes on both fields. Which action will most likely improve query performance?

A Add a single-field index on an unrelated field.

B Drop both single-field indexes and rely on collection scan.

C Create a text index on <code>status</code>.

D Create a compound index on <code>{ status: 1, date: 1 }</code>.

💡 Hint

Think about how compound indexes compare to index intersection.

A compound index on both fields can be more efficient than index intersection because it stores combined keys, reducing the need to merge results.

🔧 Debug

expert

Diagnosing Unexpected Query Performance

A query filtering on type and region is slow despite having single-field indexes on both. Which is the most likely cause?

A The single-field indexes are compound indexes, causing conflicts.

B MongoDB is not using index intersection because the query uses an $or operator incorrectly.

C The collection is sharded, disabling index intersection.

D The query uses a $text search, which ignores indexes.

💡 Hint

Consider how $or affects index usage.

MongoDB cannot use index intersection for $or queries if the conditions are on different fields in separate clauses. This can cause slower performance.