The answer is to use `ScanIndexForward: false` with a `Limit` parameter. This is correct because DynamoDB’s sort key defaults to ascending order, so the most recent timestamp entries are at the end of the sorted range; without reversing the scan direction, a query for the last hour must traverse potentially millions of older records before reaching the target data, causing high latency. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of DynamoDB query patterns for time-series optimization, where a common trap is assuming auto-scaling or DAX alone fixes poor query design—the real issue is inefficient read consumption. Remember that `ScanIndexForward: false` flips the order to retrieve newest items first, and combining it with `Limit` stops the scan as soon as the required number of recent items is found, drastically reducing both latency and read capacity. A simple memory tip: “Reverse and limit to win the time-series sprint.”
DBS-C01 Workload-Specific Database Design Practice Question
This DBS-C01 practice question tests your understanding of workload-specific database design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 is designing a time-series database for IoT sensor data using Amazon DynamoDB. Each sensor sends a reading every second. The table uses 'sensor_id' as partition key and 'timestamp' as sort key. The application queries for the last hour of data for a specific sensor. The query uses 'KeyConditionExpression' with 'timestamp' between start and end time. The table has auto-scaling enabled. However, the query latency is high. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The query is scanning from the beginning of time; use 'ScanIndexForward: false' and a 'Limit' parameter.
Option B is correct because the sort key 'timestamp' is in ascending order, so the most recent data is at the end of the sorted range. When querying for the last hour, DynamoDB may scan a large number of items before finding the recent ones, especially if the sensor has been running for a long time. Using 'ScanIndexForward: false' with a limit would reverse the order and retrieve recent items first. Option A is wrong because auto-scaling would add capacity if needed, but the issue is query design. Option C is wrong because the sort key is already 'timestamp'. Option D is wrong because DAX would reduce latency but not the underlying read consumption.
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.
✗
Enable DynamoDB Accelerator (DAX) to cache the query results.
Why it's wrong here
Does not fix the underlying query inefficiency; still consumes many RCUs.
✗
The sort key should be 'sensor_id' and partition key should be 'timestamp'.
Why it's wrong here
Would break the query pattern for a specific sensor.
✓
The query is scanning from the beginning of time; use 'ScanIndexForward: false' and a 'Limit' parameter.
Why this is correct
Reverses the sort order to retrieve recent items first, reducing scanned data.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The table does not have enough read capacity units; increase the base capacity.
Why it's wrong here
Auto-scaling would increase capacity if needed; the issue is query pattern.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Got this wrong? Here's your next step.
Identify which DBS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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: The query is scanning from the beginning of time; use 'ScanIndexForward: false' and a 'Limit' parameter. — Option B is correct because the sort key 'timestamp' is in ascending order, so the most recent data is at the end of the sorted range. When querying for the last hour, DynamoDB may scan a large number of items before finding the recent ones, especially if the sensor has been running for a long time. Using 'ScanIndexForward: false' with a limit would reverse the order and retrieve recent items first. Option A is wrong because auto-scaling would add capacity if needed, but the issue is query design. Option C is wrong because the sort key is already 'timestamp'. Option D is wrong because DAX would reduce latency but not the underlying read consumption.
What should I do if I get this DBS-C01 question wrong?
Identify which DBS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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