C100DBA Application Administration Practice Question
Exhibit
{
"ns": "app.orders",
"nScanned": 500000,
"nReturned": 1,
"execStats": {
"stage": "COLLSCAN",
"nReturned": 1
}
}Refer to the exhibit. What is the primary cause of the performance issue seen in this query plan?
⚠ Common exam trap
Candidates frequently misinterpret 'COLLSCAN' as a network latency issue rather than recognizing it as a fundamental lack of an appropriate index for the query filter.
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
✓
The query requires a full collection scan.
The exhibit shows a 'COLLSCAN', which indicates the database is scanning every document in the collection to satisfy the query. With 500,000 scanned documents returned for a single result, the query is severely inefficient. This demonstrates a lack of a supporting index, leading to high CPU and I/O usage. Identifying these inefficient queries is vital for DBAs to prevent application-wide slowdowns caused by resource exhaustion on the primary database node.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The query is hitting the primary index limit.
Why it's wrong here
There is no such thing as a 'primary index limit' in MongoDB. The performance issue here is caused by the absence of an appropriate index for the query filter, which forces the engine to perform a full collection scan rather than looking up the specific document via an index entry.
- ✓
The query requires a full collection scan.
Why this is correct
The presence of 'COLLSCAN' in the execution statistics proves that the query engine is reading every document in the collection to find a match. This is the most expensive operation in MongoDB and confirms that the current query filters do not have an associated index to speed up retrieval.
- ✗
The query is being blocked by a write lock.
Why it's wrong here
Write locks prevent other operations from accessing the collection, but they do not manifest as a 'COLLSCAN' in the explain output. The explain output describes how the query is executed, and a scan indicates a query planning issue related to indexing, not a concurrency contention issue within the database.
- ✗
The database is out of memory.
Why it's wrong here
While memory pressure can slow down queries, the execution plan shows that the engine specifically chose a full collection scan. If memory were the issue, you might see disk spill errors or high cache eviction rates, but the query plan itself would reflect an index scan if a proper index existed.
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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 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.