Multikey indexes for arrays in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of a query using a multikey index on an array field

Given a MongoDB collection products with documents like:

{ "_id": 1, "tags": ["red", "cotton", "sale"] }

and a multikey index on tags, what documents will this query return?

db.products.find({ tags: "cotton" })

MongoDB

db.products.insertMany([
  { _id: 1, tags: ["red", "cotton", "sale"] },
  { _id: 2, tags: ["blue", "wool"] },
  { _id: 3, tags: ["cotton", "new"] }
])
db.products.createIndex({ tags: 1 })
db.products.find({ tags: "cotton" })

A Documents with _id 2 only

B Documents with _id 1 and 3

C Documents with _id 1, 2, and 3

D No documents returned

💡 Hint

Think about how multikey indexes work with array fields and matching values.

The multikey index indexes each element of the array separately. So the query matches documents where the tags array contains "cotton". Documents 1 and 3 have "cotton" in their tags, so they are returned.

🧠 Conceptual

intermediate

Understanding multikey index limitations

Which of the following is a limitation of MongoDB multikey indexes on array fields?

A They cannot index arrays with more than 100 elements

B They automatically index only the first element of the array

C They do not support indexing nested arrays (arrays inside arrays)

D They require the array to be sorted before indexing

💡 Hint

Think about how MongoDB handles arrays inside arrays for indexing.

MongoDB multikey indexes do not support indexing nested arrays (arrays inside arrays). This is a known limitation. Other options are incorrect because MongoDB indexes all elements, does not require sorting, and has no strict 100 element limit.

📝 Syntax

advanced

Correct syntax to create a multikey index on an array field

Which of the following commands correctly creates a multikey index on the categories array field in MongoDB?

A db.collection.createIndex({ categories: { type: "array" } })

B db.collection.createIndex({ categories: "multikey" })

C db.collection.createIndex({ categories: [1] })

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

💡 Hint

Remember how MongoDB creates indexes on fields, including arrays.

MongoDB automatically creates a multikey index when you create a normal index on an array field. The syntax is the same as for any field: { categories: 1 }. Other options are invalid syntax or unsupported.

❓ optimization

advanced

Optimizing queries with multikey indexes on multiple array fields

Consider a collection with documents having two array fields: colors and sizes. You want to efficiently query documents where colors contains "red" and sizes contains "large".

Which index strategy is best for this query?

A Create a compound multikey index on { colors: 1, sizes: 1 }

B Create separate single-field multikey indexes on colors and sizes

C Create a text index on colors and sizes fields

D Create a hashed index on colors and sizes

💡 Hint

Think about how compound multikey indexes work with multiple array fields.

MongoDB supports compound multikey indexes on multiple array fields, which can optimize queries filtering on both fields. Separate indexes may cause inefficient query plans. Text and hashed indexes are not suitable for this type of query.

🔧 Debug

expert

Why does this query not use the multikey index?

You created a multikey index on the tags array field. But this query is slow and does not use the index:

db.products.find({ tags: { $all: ["cotton", "sale"] } })

What is the most likely reason?

A The multikey index exists but MongoDB cannot use it efficiently for $all with multiple elements

B The query uses $all with multiple values, which requires a special index type

C Multikey indexes do not support $all queries on array fields

D The index was created on a different field, not tags

💡 Hint

Consider how MongoDB uses multikey indexes with $all operator.

Multikey indexes support $all queries, but when $all has multiple elements, MongoDB may not efficiently use the index if the array elements are large or the index is not selective enough. This can cause slow queries despite the index existing.

Practice

1. What is the main purpose of a multikey index in MongoDB when applied to an array field?

easy

A. To index each element inside the array separately for faster queries

B. To store the entire array as a single indexed value

C. To prevent duplicate values inside the array

D. To convert arrays into strings before indexing

Solution

1. Step 1: Understand what a multikey index does

   A multikey index indexes each element of an array field separately, allowing efficient queries on any element inside the array.

2. Step 2: Compare options with this behavior

   Only To index each element inside the array separately for faster queries describes indexing each element separately. Other options describe incorrect behaviors.

3. Final Answer:

   To index each element inside the array separately for faster queries -> Option A

4. Quick Check:

   Multikey index = indexes array elements separately [OK]

Hint: Multikey indexes each array element, not the whole array [OK]

Common Mistakes:

• Thinking multikey indexes the whole array as one value
• Confusing multikey with unique constraints
• Assuming arrays are converted to strings before indexing

2. Which of the following is the correct syntax to create a multikey index on the tags array field in a MongoDB collection named posts?

easy

A. db.posts.createIndex({ tags: [1] })

B. db.posts.createIndex({ tags: 1 })

C. db.posts.createIndex({ tags: 'multikey' })

D. db.posts.createIndex({ tags: { $each: 1 } })

Solution

1. Step 1: Recall MongoDB index creation syntax

   To create an index on a field, use createIndex({ field: 1 }) for ascending order.

2. Step 2: Understand multikey index creation

   MongoDB automatically creates a multikey index if the field is an array. No special keyword is needed.

3. Final Answer:

   db.posts.createIndex({ tags: 1 }) -> Option B

4. Quick Check:

   Index array field normally = multikey index [OK]

Hint: Just index the array field normally; MongoDB handles multikey [OK]

Common Mistakes:

• Using special keywords like 'multikey' in index definition
• Trying to pass array syntax in index keys
• Misusing MongoDB operators inside createIndex

3. Given the collection products with documents like { _id: 1, categories: ["electronics", "gadgets"] }, what will be the result of the query db.products.find({ categories: "gadgets" }) if a multikey index exists on categories?

medium

A. [{ _id: 1, categories: ["electronics"] }]

B. [] (empty array)

C. Error: Cannot query array field

D. [{ _id: 1, categories: ["electronics", "gadgets"] }]

Solution

1. Step 1: Understand query on array field with multikey index

   The query searches for documents where the array contains "gadgets". The multikey index allows fast lookup of such elements.

2. Step 2: Check the document matching the query

   The document with _id 1 has "gadgets" in its categories array, so it matches and will be returned.

3. Final Answer:

   [{ _id: 1, categories: ["electronics", "gadgets"] }] -> Option D

4. Quick Check:

   Query array element with multikey index returns matching docs [OK]

Hint: Query matches docs with array containing the value [OK]

Common Mistakes:

• Expecting empty result because array is indexed
• Thinking query errors on array fields
• Confusing partial array matches with exact matches

4. You created a multikey index on the tags array field, but your query db.articles.find({ tags: "news" }) is slow. What is a likely reason?

medium

A. The index was created on a different field, not tags

B. Multikey indexes do not support queries on array elements

C. You must use $elemMatch to query array fields

D. MongoDB does not index array fields automatically

Solution

1. Step 1: Check index field correctness

   If the index is on a different field, queries on tags won't use the index, causing slowness.

2. Step 2: Verify multikey index behavior

   Multikey indexes do support queries on array elements without special operators like $elemMatch.

3. Final Answer:

   The index was created on a different field, not tags -> Option A

4. Quick Check:

   Index on correct field = fast query [OK]

Hint: Ensure index is on the queried array field [OK]

Common Mistakes:

• Believing multikey indexes don't support element queries
• Thinking $elemMatch is always required
• Assuming MongoDB indexes arrays automatically without createIndex

5. You have a collection users with documents containing an array field roles. You want to create a multikey index on roles but only for users who are active (active: true). Which is the correct way to create a partial multikey index for this?

hard

A. db.users.createIndex({ roles: 1 }, { condition: { active: true } })

B. db.users.createIndex({ roles: 1, active: true })

C. db.users.createIndex({ roles: 1 }, { partialFilterExpression: { active: true } })

D. db.users.createIndex({ roles: 1 }, { filter: { active: true } })

Solution

1. Step 1: Understand partial indexes in MongoDB

   Partial indexes use partialFilterExpression to index only documents matching a filter.

2. Step 2: Apply partialFilterExpression with multikey index

   Creating an index on roles with partialFilterExpression: { active: true } indexes only active users' roles.

3. Final Answer:

   db.users.createIndex({ roles: 1 }, { partialFilterExpression: { active: true } }) -> Option C

4. Quick Check:

   Use partialFilterExpression for conditional multikey index [OK]

Hint: Use partialFilterExpression option for conditional indexes [OK]

Common Mistakes:

• Using invalid options like filter or condition
• Trying to include filter in index key object
• Not knowing partialFilterExpression syntax