The answer is to use device ID as the partition key and timestamp as the sort key. This design is most efficient because it directly matches the primary access pattern: the partition key co-locates all data points for a single device on the same physical storage, while the sort key enables DynamoDB’s native range query capability, allowing you to retrieve a precise time slice using a KeyConditionExpression on timestamp between start and end. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your understanding of DynamoDB’s single-table design principles and how to avoid hot partitions by choosing a high-cardinality partition key like device ID. A common trap is to use timestamp as the partition key, which would create a hot partition for every second and force inefficient scans. Remember the memory tip: “Partition on the entity, sort on the time”—this keeps your queries fast and your costs low.
DBS-C01 Workload-Specific Database Design Practice Question
This DBS-C01 practice question tests your understanding of workload-specific database design. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company uses Amazon DynamoDB for a time-series IoT application. Each device sends a data point every second. The application queries data by device ID and timestamp range. Which table design is most efficient?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use device ID as the partition key and timestamp as the sort key.
Option D is correct because it models the access pattern directly: using device ID as the partition key ensures all data for a device is co-located, and timestamp as the sort key enables efficient range queries (e.g., Query with KeyConditionExpression on timestamp between start and end). This design avoids hot partitions and allows DynamoDB to retrieve the exact time-series slice without scanning.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 composite key of device ID and timestamp as the partition key.
Why it's wrong here
Prevents querying by device ID alone.
✗
Use device ID as the partition key and a random suffix as the sort key.
Why it's wrong here
Random sort key does not support timestamp range queries.
✗
Use timestamp as the partition key and device ID as the sort key.
Why it's wrong here
Leads to hot partitions on timestamp.
✓
Use device ID as the partition key and timestamp as the sort key.
Why this is correct
Allows efficient range queries on timestamp per device.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that a composite partition key (device ID + timestamp) is needed for uniqueness, but the trap here is that candidates forget the sort key's role in enabling range queries and instead try to force uniqueness into the partition key, which breaks the access pattern.
Detailed technical explanation
How to think about this question
DynamoDB's Query operation can only retrieve items from a single partition key value; by using device ID as the partition key, all data for that device resides in one partition, and the sort key (timestamp) is indexed as a sorted range, allowing efficient use of the Between operator. Under the hood, DynamoDB stores items in sorted order by sort key within each partition, so a range query on timestamp reads only the contiguous items needed, with O(log n) lookup complexity per partition. In a real-world IoT scenario with millions of devices, this design also supports adaptive capacity and avoids the 'hot key' problem if each device's write throughput is within its partition's limits.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use device ID as the partition key and timestamp as the sort key. — Option D is correct because it models the access pattern directly: using device ID as the partition key ensures all data for a device is co-located, and timestamp as the sort key enables efficient range queries (e.g., Query with KeyConditionExpression on timestamp between start and end). This design avoids hot partitions and allows DynamoDB to retrieve the exact time-series slice without scanning.
What should I do if I get this DBS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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