C100DBA Indexing and Performance Practice Question
When evaluating index performance, what is the impact of a 'partial index'?
⚠ Common exam trap
Test-takers frequently confuse partial indexes with sparse indexes, assuming partial indexes only omit null fields rather than applying arbitrary developer-defined filter expressions.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It allows for faster writes by indexing fewer documents.
Partial indexes allow you to index only a subset of documents that meet a specific filter condition. This reduces index size and the overhead of maintaining the index during writes, as only updates to documents matching the filter trigger an index update. This is highly effective for reducing memory usage when you only need to index documents with specific statuses, such as 'active' or 'pending', rather than the entire collection.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It forces all documents in the collection to be indexed.
Why it's wrong here
The exact opposite is true; a partial index only includes documents that satisfy the filter expression defined during index creation. This allows the database to ignore irrelevant documents, thereby saving storage space and reducing the processing power required to keep the index up to date during write operations.
- ✓
It allows for faster writes by indexing fewer documents.
Why this is correct
Because a partial index only tracks a subset of documents, fewer index updates are required when documents are inserted or modified. This directly improves write throughput, especially in collections where the vast majority of documents do not match the criteria, effectively optimizing both index size and system performance.
- ✗
It automatically makes all queries return faster.
Why it's wrong here
Partial indexes only speed up queries that include the filter criteria in their predicate. If a query does not match the partial filter, the database will ignore the index entirely. Therefore, they are not a universal performance boost, but rather a targeted tool for optimizing specific, highly frequent query patterns.
- ✗
It is only compatible with unique indexes.
Why it's wrong here
While partial indexes can be used to enforce partial uniqueness, they are not limited to unique indexes. You can create non-unique partial indexes to support any query pattern that targets a specific subset of data. They are a flexible feature designed to improve efficiency for various query types in large datasets.
About these practice questions
This C100DBA question is part of Courseiva's 222-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official MongoDB exam blueprint
This C100DBA practice question is part of Courseiva's free MongoDB certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the C100DBA exam.