C100DEV Indexing Practice Question
What is the primary function of the explain() method in MongoDB?
⚠ Common exam trap
Candidates often think explain() is used for performance tuning by modifying the query, rather than recognizing it purely as a diagnostic tool for viewing execution plans.
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
✓
To provide diagnostic information about query execution.
The explain() method provides detailed information about how a query is executed, including which index was used, the number of documents scanned, and the time taken for the operation. It is the essential diagnostic tool for database administrators to identify performance bottlenecks, verify that queries are using the intended indexes, and optimize slow-running queries by examining the execution plan and resource usage statistics provided by the database engine.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
To automatically optimize the query for faster execution.
Why it's wrong here
The explain() method is a diagnostic tool, not an automated optimizer. It reports on the execution path but does not rewrite the query or create indexes to make it faster. The developer must interpret the results and manually implement changes like adding indexes to improve the performance of the query.
- ✓
To provide diagnostic information about query execution.
Why this is correct
The explain() method returns a document containing the execution plan, including index usage, stage information, and performance statistics. This data is critical for developers and administrators to analyze how the MongoDB query planner decided to execute the operation and whether it utilized the optimal index for the request.
- ✗
To delete inefficient indexes from the collection.
Why it's wrong here
The explain() method does not perform any write operations or index management tasks. It is a read-only diagnostic command that provides insights into how the current query engine processes data requests. Deleting indexes must be performed explicitly using the dropIndex() command, not through the explain() analysis process.
- ✗
To force the query to use a specific index.
Why it's wrong here
While explain() can show which index is being used, it cannot force the use of a specific index. Forcing an index requires the hint() method, which instructs the query planner to bypass its standard decision process and use a particular index defined by the user for the query.
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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.