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.

Practice

1. What is the main purpose of the createIndex method in MongoDB?

easy

A. To speed up queries by creating an index on a field

B. To delete documents from a collection

C. To update multiple documents at once

D. To create a new collection

Solution

1. Step 1: Understand what createIndex does

   The createIndex method organizes data to make searches faster.

2. Step 2: Identify the correct purpose

   It does not delete or update documents, nor create collections, only speeds up queries.

3. Final Answer:

   To speed up queries by creating an index on a field -> Option A

4. Quick Check:

   createIndex speeds up queries [OK]

Hint: Remember: createIndex speeds up searches, not data changes [OK]

Common Mistakes:

• Confusing createIndex with delete or update operations
• Thinking createIndex creates collections
• Assuming createIndex modifies documents

2. Which of the following is the correct syntax to create an ascending 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: -1 })

D. db.collection.createIndex({ age })

Solution

1. Step 1: Recall syntax for createIndex

   The method requires an object with field names as keys and 1 or -1 as values for ascending or descending.

2. Step 2: Identify correct ascending index syntax

   { age: 1 } means ascending index on age. Other options are either descending (-1) or invalid syntax.

3. Final Answer:

   db.collection.createIndex({ age: 1 }) -> Option A

4. Quick Check:

   Ascending index uses 1 [OK]

Hint: Use 1 for ascending, -1 for descending in createIndex [OK]

Common Mistakes:

• Using -1 for ascending index
• Missing curly braces around field and order
• Passing field without object braces

3. Given the command db.users.createIndex({ email: 1 }, { unique: true }), what will happen if you try to insert two documents with the same email value?

medium

A. The index will be ignored and duplicates allowed

B. Both documents will be inserted without error

C. The second insert will fail due to duplicate key error

D. MongoDB will automatically update the first document

Solution

1. Step 1: Understand unique index effect

   Unique index prevents duplicate values in the indexed field.

2. Step 2: Predict behavior on duplicate insert

   Inserting a duplicate email will cause an error and reject the second insert.

3. Final Answer:

   The second insert will fail due to duplicate key error -> Option C

4. Quick Check:

   Unique index blocks duplicates [OK]

Hint: Unique index stops duplicate values in that field [OK]

Common Mistakes:

• Assuming duplicates are allowed with unique index
• Thinking MongoDB auto-updates duplicates
• Believing index is ignored on duplicates

4. What is wrong with this command? db.products.createIndex({ price: 'ascending' })

medium

A. The field name must be a string

B. The collection name is missing

C. createIndex cannot be used on numeric fields

D. The index order must be 1 or -1, not 'ascending'

Solution

1. Step 1: Check index order values

   MongoDB requires 1 for ascending or -1 for descending, not strings like 'ascending'.

2. Step 2: Identify error cause

   Using 'ascending' causes syntax error because it's not a valid order value.

3. Final Answer:

   The index order must be 1 or -1, not 'ascending' -> Option D

4. Quick Check:

   Index order must be 1 or -1 [OK]

Hint: Use 1 or -1 for index order, not words [OK]

Common Mistakes:

• Using string values instead of 1 or -1
• Thinking createIndex can't index numbers
• Confusing collection name with index fields

5. You want to create a compound index on lastName (ascending) and age (descending) fields, ensuring no two documents have the same combination. Which command is correct?

hard

A. db.collection.createIndex({ lastName: -1, age: 1 }, { unique: false })

B. db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true })

C. db.collection.createIndex({ lastName: 1, age: 1 }, { unique: true })

D. db.collection.createIndex({ lastName: 'asc', age: 'desc' }, { unique: true })

Solution

1. Step 1: Understand compound index syntax

   Compound index uses an object with multiple fields and their order: 1 for ascending, -1 for descending.

2. Step 2: Check uniqueness and order

   db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true }) matches ascending lastName (1), descending age (-1), and unique true to prevent duplicates.

3. Step 3: Verify other options

   db.collection.createIndex({ lastName: -1, age: 1 }, { unique: false }) reverses order and is not unique; C uses ascending age instead of descending; D uses invalid string values.

4. Final Answer:

   db.collection.createIndex({ lastName: 1, age: -1 }, { unique: true }) -> Option B

5. Quick Check:

   Compound unique index uses correct order and unique:true [OK]

Hint: Use 1/-1 for order and unique:true for no duplicates [OK]

Common Mistakes:

• Using strings 'asc'/'desc' instead of 1/-1
• Setting unique:false when uniqueness is needed
• Mixing ascending and descending order incorrectly