20+ practice questions focused on Indexing — one of the most tested topics on the MongoDB Certified Associate Developer exam. Each question includes a detailed explanation so you learn why the right answer is correct.
Start Indexing PracticeAn application frequently queries a collection using equality conditions on 'status' and 'category', followed by a range filter on 'created_at', and sorts the final results by 'priority' descending. How should you design a compound index for optimal performance according to the ESR rule?
Explanation: The Equality, Sort, and Range rule dictates that fields with equality matches must come first, followed by the sort fields, and finally the range filter fields. This precise positioning prevents unnecessary in-memory sorts and allows MongoDB to use index bounds directly for range scans, maximizing query efficiency and minimizing traversal overhead in high-throughput production environments.
An application queries an array field named 'tags' containing strings for product categorization. Queries frequently search for documents matching multiple tags simultaneously. Which index strategy optimizes this query workload?
Explanation: Creating a multikey index on the tags array allows MongoDB to index every element of the array individually. When queries search for multiple elements, the query planner can efficiently intersect or utilize the multikey entries to narrow down candidate documents without performing full collection scans. This is the foundational pattern for querying any array field effectively in MongoDB applications.
A developer needs to ensure that the 'email' field in a 'users' collection is unique across all documents. Which TWO statements accurately describe the behavior or requirements of unique indexes in MongoDB?
Explanation: Unique indexes ensure that the indexed fields do not store duplicate values, which is critical for data integrity. They handle null values by treating them as a single unique value, meaning only one document can have a null 'email' unless a sparse or partial index is used. Understanding these behaviors prevents common errors when dealing with optional fields and data migration.
You have a collection with 10 million documents. Most documents have a 'status' of 'processed', but about 1% have a 'status' of 'pending'. You frequently query for 'pending' documents sorted by 'creation_date'. Which indexing strategy provides the best balance between write performance and query efficiency?
Explanation: Partial indexes allow you to index only a subset of documents that meet a specific filter expression. In this scenario, indexing only 'pending' documents significantly reduces the index size and the overhead of updating the index during writes for 'processed' documents. This targeted approach is more efficient than a full index when a small fraction of data is frequently accessed.
Refer to the exhibit. Based on the explain output provided for a query on the 'logs' collection, which of the following is true about the query performance?
Explanation: The explain output shows an ideal scenario where the number of keys examined and documents examined exactly match the number of documents returned. This indicates that the index is highly selective and perfectly matches the query's filtering criteria. A 'totalKeysExamined' to 'nReturned' ratio of 1:1 is the gold standard for index efficiency in MongoDB.
+15 more Indexing questions available
Practice all Indexing questions1. Baseline your knowledge
Start with 10 questions to gauge your current understanding of Indexing. This tells you whether you need a concept refresher or just practice.
2. Review every explanation
For each question — right or wrong — read the full explanation. Understanding why an answer is correct is more valuable than knowing the answer itself.
3. Focus on exam traps
Indexing questions on the C100DEV frequently use trap wording. Look for subtle differences in answers that test your precision, not just general knowledge.
4. Reach 80% consistently
Do repeated sessions until you score 80%+ three times in a row. Then move to mixed-mode practice to test cross-topic recall under realistic conditions.
The exact number varies per candidate. Indexing is tested as part of the MongoDB Certified Associate Developer blueprint. Practicing with targeted Indexing questions ensures you can handle any format or difficulty that appears.
Yes. Courseiva provides free C100DEV practice questions across all exam topics and domains. The platform includes topic-based practice, mock exams, missed-question review, bookmarked questions, and readiness tracking — no account required.
Difficulty is subjective, but Indexing is a high-priority exam concept tested in multiple ways — direct recall, scenario analysis, and command-output interpretation. Consistent practice is the best way to build confidence.
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