C100DBA Sharding Practice Question
An application executes a query on a sharded collection that does not include the shard key. What is the impact on the cluster performance?
⚠ Common exam trap
Candidates often confuse a scatter-gather query with a targeted query, mistakenly thinking the mongos router sends the request only to the shard containing the matching range.
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 mongos will perform a 'scatter-gather' operation across all shards.
When a query does not include the shard key, the mongos router cannot determine which shard contains the requested data. As a result, it must broadcast the query to every shard in the cluster. This 'scatter-gather' operation increases resource consumption across all nodes and can lead to significant latency and scalability issues as the cluster grows.
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 will be rejected by the mongos as an invalid operation.
Why it's wrong here
MongoDB allows queries that do not include the shard key. The mongos router will still attempt to execute the query by sending it to all shards. While inefficient, it is a valid operation, provided the query is otherwise syntactically correct and the user has the necessary permissions.
- ✗
The query will only be sent to the primary shard of the database.
Why it's wrong here
The primary shard only stores unsharded collections for a database. For a sharded collection, the data is distributed across many shards. A query without the shard key must be sent to every shard that might contain data for that collection, not just the database's primary shard.
- ✓
The mongos will perform a 'scatter-gather' operation across all shards.
Why this is correct
In a scatter-gather operation, the mongos sends the query to all shards simultaneously. It then waits for all shards to respond, merges the results, and returns them to the application. This process is expensive in terms of network overhead and CPU usage across the entire cluster.
- ✗
The cluster will automatically create a new index to support the query.
Why it's wrong here
MongoDB does not automatically create indexes based on query patterns. Indexing is an administrative task. Furthermore, even with an index on the queried field, the mongos still needs the shard key to target specific shards; otherwise, the index lookup must still occur on every shard.
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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.