Single field index in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Effect of Single Field Index on Query Performance

Consider a MongoDB collection users with a single field index on age. Which query will most likely use this index to improve performance?

MongoDB

db.users.createIndex({ age: 1 })
db.users.find({ age: { $gt: 30 } })

A The query <code>db.users.find({ age: { $gt: 30 } })</code> uses the index and returns all users older than 30.

B The query <code>db.users.find({ age: { $exists: true } })</code> uses the index to find all users with an age field.

C The query <code>db.users.find({ name: 'Alice' })</code> uses the index on age to find users named Alice.

D The query <code>db.users.find({ age: { $lt: 20 }, name: 'Bob' })</code> uses the index on age and name fields.

💡 Hint

Indexes help queries that filter on the indexed field.

The index on age helps queries filtering by age. Query C filters by age and uses the index. Query B filters by name, which is not indexed. Query A uses $exists which does not efficiently use the index. Query D filters by age and name, but only age is indexed, so the index helps only partially.

🧠 Conceptual

intermediate

Understanding Single Field Index Uniqueness

What is true about a single field index created with { field: 1 } in MongoDB?

A It creates a non-unique index that speeds up queries filtering by that field.

B It only indexes documents where the field exists.

C It creates a unique index by default, preventing duplicate values in the field.

D It automatically indexes all fields in the collection.

💡 Hint

Think about default behavior of indexes in MongoDB.

By default, single field indexes are non-unique unless specified. They speed up queries filtering by the indexed field. They do not index all fields or only documents with the field.

📝 Syntax

advanced

Correct Syntax to Create a Single Field Index

Which of the following commands correctly creates a single field ascending index on the email field in MongoDB?

A db.collection.createIndex(['email', 1])

B db.collection.createIndex(email: 1)

C db.collection.createIndex({ 'email' => 1 })

D db.collection.createIndex({ email: 1 })

💡 Hint

Remember the syntax for specifying fields in an object.

Option D uses the correct syntax: an object with field name as key and 1 for ascending order. Option D is missing braces. Option D uses invalid arrow syntax. Option D uses an array which is invalid.

❓ optimization

advanced

Choosing the Best Index for Query Optimization

You have a collection with fields category and price. You often run queries filtering only by category. Which index is best to optimize these queries?

A A compound index on <code>category</code> and <code>price</code>.

B A single field index on <code>category</code> only.

C A single field index on <code>price</code> only.

D No index is needed for filtering by <code>category</code>.

💡 Hint

Indexes should match the fields used in query filters.

Since queries filter only by category, a single field index on category is best. Compound indexes help when filtering by multiple fields in order. Index on price alone won't help filtering by category. No index means slower queries.

🔧 Debug

expert

Diagnosing Query Performance Issue with Single Field Index

A MongoDB collection has a single field index on status. A query db.orders.find({ status: { $in: ['shipped', 'delivered'] } }) is slow. What is the most likely reason?

A The <code>$in</code> operator cannot use single field indexes.

B The index on <code>status</code> is not being used because the query uses an array.

C The index exists but the query is slow because of missing index statistics or outdated index.

D The collection is too large and needs a compound index including <code>status</code> and <code>date</code>.

💡 Hint

Think about index usage and maintenance in MongoDB.

The $in operator can use single field indexes. The index is likely used but query is slow due to outdated statistics or fragmentation. Large collections may need compound indexes but that is not the main cause here.