DBS-C01 Workload-Specific Database Design Practice Question
A company uses Amazon DynamoDB to store IoT sensor data. Each sensor writes a record every second, and the application needs to query the last 24 hours of data for a specific sensor. The query must be very fast. Which table design and query pattern will minimize cost and latency?
⚠ Common exam trap
Candidates often choose Option C, mistakenly thinking that timestamp as a partition key provides global time ordering, but this actually scatters sensor data across partitions, making per-sensor queries impossible without a full scan.
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
✓
Use a composite primary key: partition key = sensor ID, sort key = timestamp
Using a composite primary key with partition key = sensor ID and sort key = timestamp allows DynamoDB to efficiently query all items for a specific sensor in a single partition, using the Query API with a sort key condition on timestamp. This design ensures fast, targeted reads without scanning, minimizing read capacity units and latency.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use a simple primary key (sensor ID) and scan the table filtering by timestamp
Why it's wrong here
Scan is expensive and slow for large tables.
- ✓
Use a composite primary key: partition key = sensor ID, sort key = timestamp
Why this is correct
This allows efficient range queries on timestamp for a sensor.
- ✗
Use a composite primary key: partition key = timestamp, sort key = sensor ID
Why it's wrong here
This would scatter data across partitions but still require a scan for a sensor.
- ✗
Use a simple primary key (sensor ID) and a global secondary index on timestamp
Why it's wrong here
GSI adds cost and querying by sensor ID then filtering by timestamp is less efficient.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DBS-C01 practice question is part of Courseiva's free Amazon Web Services 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 DBS-C01 exam.