createIndex method in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this createIndex command do?

Consider the following MongoDB command:

db.users.createIndex({ email: 1 }, { unique: true })

What is the effect of this command on the users collection?

MongoDB

db.users.createIndex({ email: 1 }, { unique: true })

A Creates a descending index on the email field but allows duplicate email values.

B Removes all documents with duplicate email values from the collection.

C Creates a text index on the email field for full-text search.

D Creates an ascending index on the email field and enforces unique email values in the collection.

💡 Hint

Think about what the 1 and unique: true mean in the index creation.

The 1 means ascending order index on the email field. The unique: true option enforces that no two documents can have the same email value.

❓ query_result

intermediate

What is the output of this createIndex command?

Given the command:

db.orders.createIndex({ orderDate: -1 })

What kind of index is created on the orderDate field?

MongoDB

db.orders.createIndex({ orderDate: -1 })

A A descending index on orderDate.

B A hashed index on orderDate.

C An ascending index on orderDate.

D A compound index on orderDate and another field.

💡 Hint

The value -1 indicates the index order direction.

In MongoDB, -1 means descending order index on the specified field.

📝 Syntax

advanced

Which createIndex command is syntactically correct?

Choose the correct MongoDB createIndex command syntax to create a compound index on fields firstName (ascending) and lastName (descending):

A db.collection.createIndex({ firstName: 'asc', lastName: 'desc' })

B db.collection.createIndex([ firstName: 1, lastName: -1 ])

C db.collection.createIndex({ firstName: 1, lastName: -1 })

D db.collection.createIndex({ firstName => 1, lastName => -1 })

💡 Hint

Remember the syntax for specifying fields and order in createIndex.

The correct syntax uses an object with field names as keys and 1 or -1 as values for ascending or descending order.

❓ optimization

advanced

Which createIndex option improves query performance for frequent searches on a field with many duplicate values?

You have a collection with a field status that has only a few distinct values repeated many times. Which index type should you create to optimize queries filtering by status?

A Create a unique index on the status field.

B Create a hashed index on the status field.

C Create a sparse index on the status field.

D Create a multikey index on the status field.

💡 Hint

Think about how hashed indexes distribute values for equality queries.

Hashed indexes are good for fields with many duplicate values and optimize equality queries by hashing the field values.

🔧 Debug

expert

Why does this createIndex command fail?

You run this command:

db.products.createIndex({ price: 1 }, { unique: true })

But it fails with an error about duplicate keys. Why?

MongoDB

db.products.createIndex({ price: 1 }, { unique: true })

A Because there are multiple documents with the same price value violating the unique constraint.

B Because the createIndex method requires a second field for uniqueness.

C Because unique indexes cannot be created on numeric fields.

D Because the price field cannot be indexed in ascending order.

💡 Hint

Unique indexes require all indexed values to be different.

The error occurs because the collection has duplicate price values, which violates the unique index requirement.