- A
Use DynamoDB Time to Live (TTL) to expire old data
Correct. TTL reduces data volume, improving query performance and reducing storage and read costs.
- B
Create a local secondary index on device_id and timestamp
Why wrong: Incorrect. The base table already has device_id as partition key and timestamp as sort key, enabling efficient range queries. Adding an LSI on the same attributes is redundant and does not improve performance or reduce cost.
- C
Enable DynamoDB Accelerator (DAX)
Why wrong: Incorrect. DAX is an in-memory cache that improves read performance but adds cost for the DAX cluster. It does not reduce overall cost.
- D
Create a global secondary index on timestamp
Why wrong: Incorrect. A GSI on timestamp allows queries by timestamp only, not by device_id. A query for a specific device_id would still require a full scan of the index, which is inefficient.
Using Local Secondary Index for Time Range Queries in DynamoDB
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. A key principle to apply: time to Live (TTL). 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.
An IoT company ingests sensor data into Amazon DynamoDB. The data has a partition key of device_id and sort key of timestamp. Queries often filter by device_id and a date range. Which design pattern improves query performance and reduces cost?
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 DynamoDB Time to Live (TTL) to expire old data
Option A is correct because DynamoDB Time to Live (TTL) automatically expires and deletes old sensor data. By removing obsolete items, the table size is reduced, leading to faster queries (less data to scan) and lower storage costs. Additionally, fewer read capacity units are consumed because queries process fewer items. This pattern both improves query performance and reduces cost for time-series data where queries focus on recent date ranges.
Key principle: Time to Live (TTL)
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 DynamoDB Time to Live (TTL) to expire old data
Why this is correct
Correct. TTL reduces data volume, improving query performance and reducing storage and read costs.
Related concept
Time to Live (TTL)
- ✗
Create a local secondary index on device_id and timestamp
Why it's wrong here
Incorrect. The base table already has device_id as partition key and timestamp as sort key, enabling efficient range queries. Adding an LSI on the same attributes is redundant and does not improve performance or reduce cost.
- ✗
Enable DynamoDB Accelerator (DAX)
Why it's wrong here
Incorrect. DAX is an in-memory cache that improves read performance but adds cost for the DAX cluster. It does not reduce overall cost.
- ✗
Create a global secondary index on timestamp
Why it's wrong here
Incorrect. A GSI on timestamp allows queries by timestamp only, not by device_id. A query for a specific device_id would still require a full scan of the index, which is inefficient.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates overlook TTL as a performance tool. They often assume an index is needed when the base table already supports the query pattern. TTL not only saves storage cost but also speeds up queries by reducing the data set.
Detailed technical explanation
How to think about this question
A local secondary index (LSI) maintains the same partition key as the base table (device_id) but allows a different sort key (timestamp), enabling efficient range queries (e.g., BETWEEN, >, <) on timestamp per device. LSIs are stored in the same partition as the base table data, ensuring strong consistency and low latency, but they consume additional write capacity for index updates. In practice, this pattern is ideal for time-series IoT data where you need to retrieve all readings for a specific device within a given date range without scanning unrelated partitions.
KKey Concepts to Remember
- Time to Live (TTL)
- Base table partition and sort key
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
Time to Live (TTL)
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Review time to Live (TTL), then practise related DBS-C01 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this DBS-C01 question test?
Workload-Specific Database Design — This question tests Workload-Specific Database Design — Time to Live (TTL).
What is the correct answer to this question?
The correct answer is: Use DynamoDB Time to Live (TTL) to expire old data — Option A is correct because DynamoDB Time to Live (TTL) automatically expires and deletes old sensor data. By removing obsolete items, the table size is reduced, leading to faster queries (less data to scan) and lower storage costs. Additionally, fewer read capacity units are consumed because queries process fewer items. This pattern both improves query performance and reduces cost for time-series data where queries focus on recent date ranges.
What should I do if I get this DBS-C01 question wrong?
Review time to Live (TTL), then practise related DBS-C01 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Time to Live (TTL)
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Last reviewed: Jun 24, 2026
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