When not to index in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

When should you avoid indexing a field in MongoDB?

Which situation is NOT a good reason to create an index on a MongoDB collection field?

A The field contains unique values that need to be enforced.

B The field is frequently updated with new values.

C The field is often used in query filters to find documents.

D The field is used in sorting query results.

💡 Hint

Think about how indexes affect write operations and storage.

Indexing fields that are frequently updated can slow down writes because the index must be updated each time. It's better to avoid indexing such fields unless necessary.

❓ query_result

intermediate

Effect of indexing a low-cardinality field

Consider a MongoDB collection with a field status that only has two possible values: 'active' or 'inactive'. What is the likely effect of creating an index on this status field?

A The index will greatly improve query speed for filtering by status.

B The index will cause errors when querying the collection.

C The index will have little benefit and may slow down writes.

D The index will reduce the collection size significantly.

💡 Hint

Think about how selective the index is when the field has few distinct values.

Indexes on low-cardinality fields (few distinct values) usually do not improve query performance much and add overhead on writes and storage.

📝 Syntax

advanced

Identify the incorrect index creation command

Which MongoDB command will cause a syntax error when trying to create an index?

MongoDB

db.users.createIndex({ age: 'ascending' })

A db.users.createIndex({ age: 'ascending' })

B db.users.createIndex({ age: 1 })

C )} 1 :ega {(xednIetaerc.sresu.bd

D db.users.createIndex({ age: -1 })

💡 Hint

Check the valid values for index direction in MongoDB.

MongoDB accepts 1 or -1 for ascending or descending indexes. Strings like 'asc' or 'ascending' are invalid and cause syntax errors.

❓ optimization

advanced

Choosing not to index a frequently updated field

You have a MongoDB collection where the lastLogin field is updated every time a user logs in. You also query users by lastLogin date range sometimes. What is the best indexing strategy?

A Do not index <code>lastLogin</code> because frequent updates will slow writes.

B Create a compound index including <code>lastLogin</code> and <code>userId</code>.

C Create a single-field index on <code>lastLogin</code> to speed up queries.

D Create a TTL index on <code>lastLogin</code> to automatically delete old records.

💡 Hint

Consider the trade-off between read speed and write performance.

Indexing a field that is updated very often can degrade write performance significantly. If queries on that field are infrequent, it's better not to index it.

🔧 Debug

expert

Why does this index not improve query performance?

A developer created an index on the category field of a MongoDB collection. The category field contains many repeated values. Queries filtering by category are still slow. What is the most likely reason?

A The queries do not use the <code>category</code> field in their filter.

B The index was created with the wrong syntax and is not active.

C The collection is too small for indexes to have any effect.

D The index is on a low-cardinality field, so it is not selective enough to speed up queries.

💡 Hint

Think about how index selectivity affects query speed.

Indexes on fields with many repeated values (low cardinality) do not help much because the database still scans many documents. This makes queries slow despite the index.