How MongoDB scans documents - Practice Exercises

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this MongoDB query scan?

Consider a collection users with documents containing age and name. The query db.users.find({ age: { $gt: 30 } }) is run without any indexes. What kind of scan does MongoDB perform and what documents are returned?

MongoDB

db.users.find({ age: { $gt: 30 } })

A MongoDB performs a collection scan and returns all documents where age is greater than 30.

B MongoDB uses an index scan and returns documents where age is less than or equal to 30.

C MongoDB performs a collection scan but returns no documents because no index exists.

D MongoDB uses an index scan and returns all documents regardless of age.

💡 Hint

Think about what happens when no index supports the query condition.

Without an index on the age field, MongoDB must scan every document in the collection (collection scan) to find those where age is greater than 30.

🧠 Conceptual

intermediate

How does MongoDB use indexes to scan documents?

If a collection has an index on the status field, what type of scan does MongoDB perform when running db.orders.find({ status: 'shipped' })?

A MongoDB performs a random scan of documents.

B MongoDB performs an index scan using the status index to quickly find matching documents.

C MongoDB performs a full text search scan.

D MongoDB performs a collection scan ignoring the index.

💡 Hint

Indexes help MongoDB find documents faster by scanning only relevant entries.

When an index exists on the queried field, MongoDB uses an index scan to efficiently locate matching documents without scanning the entire collection.

📝 Syntax

advanced

Which MongoDB query syntax will cause an error when scanning documents?

Identify the query that will cause a syntax error when scanning documents in MongoDB.

A db.products.find({ price: { $lt: 100 } })

B db.products.find({ category: 'books' })

C db.products.find({ price: { $gte: 50 } })

D db.products.find({ $price: { $lt: 100 } })

💡 Hint

Check the use of $ sign in field names.

Field names in queries cannot start with a $ unless it is an operator. Using $price as a field name causes a syntax error.

❓ optimization

advanced

Which query will result in the most efficient document scan?

Given a collection with an index on category and price, which query will use the index most efficiently to scan documents?

A db.items.find({ category: 'electronics', price: { $lt: 500 } })

B db.items.find({ price: { $lt: 500 } })

C db.items.find({ category: 'electronics' })

D db.items.find({ name: 'laptop' })

💡 Hint

Using multiple indexed fields in a query improves scan efficiency.

Querying on both indexed fields allows MongoDB to use a compound index scan, reducing the number of documents scanned.

🔧 Debug

expert

Why does this MongoDB query scan all documents despite an index existing?

A collection has an index on status. The query db.orders.find({ status: { $in: ['shipped', 'delivered'] } }) scans all documents instead of using the index. Why?

A The query uses a non-covered index scan causing a collection scan fallback.

B The index is not a multikey index and cannot support $in queries.

C The index is not being used because the query is not selective enough.

D The $in operator disables index usage causing a collection scan.

💡 Hint

Consider how MongoDB decides to use indexes based on query selectivity.

If the query matches a large portion of documents, MongoDB may choose a collection scan over an index scan for efficiency.