C100DEV Indexing Practice Question
What is the primary benefit of using a partial index in MongoDB?
⚠ Common exam trap
Candidates often confuse partial indexes with sparse indexes, mistakenly believing partial indexes require the indexed field itself to be absent rather than evaluating an explicit filter expression.
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 reduces the index size and resource usage.
Partial indexes allow you to index only a subset of documents that meet a specific filter expression. This reduces the index size, resulting in lower memory and storage consumption. By ignoring unnecessary documents, the database avoids wasting resources on indexing data that is never or rarely queried. This optimization is particularly beneficial for large collections where only a small percentage of data is active or frequently accessed, leading to significant performance gains.
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 to be indexed.
Why it's wrong here
A partial index does the exact opposite of forcing all documents to be indexed. It only indexes documents that satisfy a specified filter expression. This selective indexing approach is the defining characteristic of partial indexes, designed to save resources and improve efficiency by ignoring non-matching documents in the collection.
- ✓
It reduces the index size and resource usage.
Why this is correct
By limiting the indexed documents to a specific subset, partial indexes significantly reduce the overall size of the index. Smaller indexes occupy less RAM in the WiredTiger cache and consume less disk space, which directly leads to improved performance for write operations and faster overall query execution for specific subsets.
- ✗
It prevents duplicates across the entire collection.
Why it's wrong here
While a unique partial index can enforce uniqueness for a subset, it does not prevent duplicates across the entire collection for documents that do not meet the filter criteria. The uniqueness constraint only applies to the documents that are included in the index, leaving other documents completely unconstrained by this rule.
- ✗
It is required for all compound indexes.
Why it's wrong here
Partial indexes are an optional feature for any index type, including compound indexes. They are not required for any index to function. Developers choose to implement them only when they need to optimize storage or performance for a specific subset of data, not as a mandatory requirement for standard indexing.
About these practice questions
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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 C100DEV 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 C100DEV exam.