Sparse indexes in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a sparse index on the field "email".

MongoDB

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

Drag options to blanks, or click blank then click option'

Asparse

Bbackground

Cunique

DexpireAfterSeconds

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'unique' instead of 'sparse' causes errors if duplicates exist.

Omitting the sparse option creates a normal index including all documents.

The sparse option creates an index that only includes documents where the indexed field exists.

2fill in blank

medium

Complete the code to find documents where the "phone" field exists using the sparse index.

MongoDB

db.contacts.find({ [1]: { $exists: true } })

Drag options to blanks, or click blank then click option'

Aphone

Bemail

Caddress

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Querying on a different field than the sparse index.

Using $exists: false instead of true.

To use the sparse index on the phone field, query documents where phone exists.

3fill in blank

hard

Fix the error in the code to create a sparse index on "username".

MongoDB

db.accounts.createIndex({ username: 1 }, { [1]: true })

Drag options to blanks, or click blank then click option'

Aunique

BexpireAfterSeconds

Cbackground

Dsparse

Attempts:

3 left

💡 Hint

Common Mistakes

Using string 'true' instead of boolean true.

Using an unrelated option name.

The sparse option must be a boolean true, not a string.

4fill in blank

hard

Fill both blanks to create an index on "profile.age" and make it run in the background.

MongoDB

db.users.createIndex({ [1]: 1 }, { [2]: true })

Drag options to blanks, or click blank then click option'

Aprofile.age

Bsparse

Cbackground

Dunique

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'sparse' instead of 'background' for the second option.

Using 'unique' which is unrelated here.

Use profile.age as the field and background: true to create the index without blocking operations.

5fill in blank

hard

Fill all three blanks to create a sparse, unique index on "email" with background creation.

MongoDB

db.customers.createIndex({ [1]: 1 }, { [2]: true, [3]: true, [4]: true })

Drag options to blanks, or click blank then click option'

Aemail

Bunique

Csparse

Dbackground

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing sparse and unique options.

Omitting background option.

This creates an index on email that is both unique and sparse, running in the background.

Practice

1. What is the main purpose of a sparse index in MongoDB?

easy

A. To index only documents that contain the indexed field

B. To index all documents regardless of field presence

C. To create a backup of the database

D. To speed up writes by disabling indexing

Solution

1. Step 1: Understand sparse index behavior

   Sparse indexes only include documents that have the indexed field present.

2. Step 2: Compare with other index types

   Unlike regular indexes, sparse indexes skip documents missing the field, saving space and improving query speed on sparse data.

3. Final Answer:

   To index only documents that contain the indexed field -> Option A

4. Quick Check:

   Sparse index = indexes only documents with the field [OK]

Hint: Sparse indexes skip documents missing the field [OK]

Common Mistakes:

• Thinking sparse indexes index all documents
• Confusing sparse indexes with backups
• Assuming sparse indexes disable indexing

2. Which of the following is the correct way to create a sparse index on the field email in MongoDB?

easy

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

B. db.collection.createIndex({ email: 1 }, { background: true })

C. db.collection.createIndex({ email: 1 }, { sparse: true })

D. db.collection.createIndex({ email: 1 }, { expireAfterSeconds: 3600 })

Solution

1. Step 1: Identify sparse index syntax

   The option { sparse: true } is used to create a sparse index in MongoDB.

2. Step 2: Verify the correct command

   The command db.collection.createIndex({ email: 1 }, { sparse: true }) correctly creates a sparse index on the email field.

3. Final Answer:

   db.collection.createIndex({ email: 1 }, { sparse: true }) -> Option C

4. Quick Check:

   Sparse index creation uses { sparse: true } [OK]

Hint: Use { sparse: true } option to create sparse indexes [OK]

Common Mistakes:

• Using unique instead of sparse option
• Confusing background with sparse
• Using expireAfterSeconds for sparse index

3. Given a collection with documents:

{ _id: 1, name: "Alice", age: 30 }

{ _id: 2, name: "Bob" }

{ _id: 3, name: "Charlie", age: 25 }

and a sparse index on age, what documents will be returned by db.collection.find({ age: { $gte: 25 } })?

medium

A. Documents with _id 1 and 3 only

B. Documents with _id 1, 2, and 3

C. Documents with _id 2 only

D. No documents returned

Solution

1. Step 1: Understand sparse index effect on query

   The sparse index on age only indexes documents that have the age field. Document with _id 2 has no age field, so it is not indexed.

2. Step 2: Evaluate the query filter

   The query { age: { $gte: 25 } } matches documents where age is 25 or more. Documents with _id 1 (age 30) and _id 3 (age 25) match. Document _id 2 is excluded because it has no age field.

3. Final Answer:

   Documents with _id 1 and 3 only -> Option A

4. Quick Check:

   Sparse index excludes docs missing field, so only _id 1 and 3 match [OK]

Hint: Sparse index excludes docs missing field, so only those with field match [OK]

Common Mistakes:

• Including documents without the indexed field
• Assuming all documents are returned
• Confusing sparse index with regular index

4. You created a sparse index on phone field but your queries on phone are slow. What is a likely cause?

medium

A. Sparse indexes index all documents regardless of field presence

B. Sparse indexes always slow down queries

C. You forgot to create the index

D. Many documents do not have the phone field, so index is small and not used

Solution

1. Step 1: Analyze sparse index behavior

   Sparse indexes only index documents with the field. If many documents lack the phone field, the index is small.

2. Step 2: Understand query performance impact

   If the index is small or not selective, MongoDB may choose not to use it, causing slower queries.

3. Final Answer:

   Many documents do not have the phone field, so index is small and not used -> Option D

4. Quick Check:

   Sparse index small if field missing in many docs, may not speed queries [OK]

Hint: Sparse index small if many docs lack field, may not improve speed [OK]

Common Mistakes:

• Assuming sparse indexes always slow queries
• Forgetting to create the index
• Thinking sparse indexes index all documents

5. You want to create an index on the status field, but only for documents where status exists and is not null. Which MongoDB index type should you use and why?

hard

A. TTL index, because it removes documents after a time

B. Sparse index, because it indexes only documents with the field present

C. Regular index, because it indexes all documents regardless of field

D. Unique index, because it enforces unique values

Solution

1. Step 1: Identify index requirement

   You want to index only documents where status exists and is not null, skipping documents without status.

2. Step 2: Match index type to requirement

   Sparse indexes index only documents that have the indexed field, which fits the requirement perfectly.

3. Final Answer:

   Sparse index, because it indexes only documents with the field present -> Option B

4. Quick Check:

   Use sparse index to index only docs with the field [OK]

Hint: Use sparse index to index only documents with the field present [OK]

Common Mistakes:

• Choosing regular index which indexes all documents
• Confusing TTL or unique indexes with sparse
• Not understanding sparse index skips missing fields