C100DBA Indexing and Performance Practice Question
What is the primary benefit of using a Covered Query in MongoDB?
⚠ Common exam trap
Candidates often confuse covered queries with regular index scans, mistakenly believing that any query using an index avoids disk reads. However, fetching documents that are projected outside the index still requires reading from disk.
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 avoids reading the full document from disk.
A covered query is one where all the fields in the query projection exist within the index itself. Because the database can retrieve all necessary data directly from the index tree without needing to fetch the actual document from disk, it drastically reduces I/O. This is one of the most effective ways to optimize read performance for high-frequency queries in large collections.
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 the data to stay in the WiredTiger cache.
Why it's wrong here
Covered queries do not influence the WiredTiger cache eviction policy. While they rely on index data, which is typically cached, the primary benefit is the elimination of the FETCH stage and document retrieval, not the management of the memory cache itself during the query execution process.
- ✓
It avoids reading the full document from disk.
Why this is correct
When an index contains all fields required by a query, MongoDB returns the data directly from the index. This removes the need to perform a costly disk fetch to retrieve the full document, significantly reducing I/O operations and improving the overall latency and throughput of the query.
- ✗
It automatically compresses all returned data.
Why it's wrong here
Compression is handled by the WiredTiger storage engine at the block level on disk. A covered query does not change how data is compressed; its primary performance advantage is the avoidance of the FETCH stage, not any specific interaction with the storage engine's internal compression algorithms.
- ✗
It eliminates the need for any index on the collection.
Why it's wrong here
A covered query requires at least one index that contains all requested fields. Without an index, the query would result in a collection scan, making it impossible to cover the query. The index is the foundational component that makes covering a query possible in the first place.
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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
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