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.
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?
Use DynamoDB Time to Live (TTL) to expire old data
Why wrong: TTL is for data lifecycle, not query performance.
Create a local secondary index on device_id and timestamp
LSI allows efficient querying by device_id and sort key.
Enable DynamoDB Accelerator (DAX)
Why wrong: Does not reduce scanned data for range queries.
Create a global secondary index on timestamp
Why wrong: Does not filter by device_id.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Create a local secondary index on device_id and timestamp
Option B is correct because a local secondary index (LSI) on device_id and timestamp allows efficient querying by device_id and a date range without scanning the entire table. Since the base table already uses device_id as the partition key, the LSI shares the same partition key but provides an alternate sort key (timestamp), enabling range queries on timestamp for each device_id. This reduces read capacity consumption and improves latency by avoiding full table scans or expensive filter expressions.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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 it's wrong here
TTL is for data lifecycle, not query performance.
Create a local secondary index on device_id and timestamp
Why this is correct
LSI allows efficient querying by device_id and sort key.
Related concept
Read the scenario before looking for a memorised answer.
Enable DynamoDB Accelerator (DAX)
Why it's wrong here
Does not reduce scanned data for range queries.
Create a global secondary index on timestamp
Why it's wrong here
Does not filter by device_id.
Common exam traps
The trap here is that candidates often confuse local secondary indexes (LSI) with global secondary indexes (GSI), assuming a GSI on timestamp alone is sufficient, but without device_id as a partition key in the index, the query cannot efficiently isolate a single device's data.
Detailed technical explanation
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..
The correct answer is: Create a local secondary index on device_id and timestamp — Option B is correct because a local secondary index (LSI) on device_id and timestamp allows efficient querying by device_id and a date range without scanning the entire table. Since the base table already uses device_id as the partition key, the LSI shares the same partition key but provides an alternate sort key (timestamp), enabling range queries on timestamp for each device_id. This reduces read capacity consumption and improves latency by avoiding full table scans or expensive filter expressions.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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