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Question 165 of 1,730

Workload-Specific Database Design →hardMultiple ChoiceObjective-mapped

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 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.

A
Enable DynamoDB Accelerator (DAX) to cache the query results.
Why wrong: Does not fix the underlying query inefficiency; still consumes many RCUs.
B
The sort key should be 'sensor_id' and partition key should be 'timestamp'.
Why wrong: Would break the query pattern for a specific sensor.
C
The query is scanning from the beginning of time; use 'ScanIndexForward: false' and a 'Limit' parameter.
Reverses the sort order to retrieve recent items first, reducing scanned data.
D
The table does not have enough read capacity units; increase the base capacity.
Why wrong: Auto-scaling would increase capacity if needed; the issue is query pattern.

Full breakdown with real-world context →

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

✓

The query is scanning from the beginning of time; use 'ScanIndexForward: false' and a 'Limit' parameter.

Option C is correct because the high query latency is most likely due to the query scanning all historical data for the sensor before applying the filter on the sort key. By default, DynamoDB queries return results in ascending order of the sort key, and without `ScanIndexForward: false` and a `Limit` parameter, the query may process many irrelevant items before reaching the last hour of data. Setting `ScanIndexForward: false` returns the most recent items first, and adding a `Limit` reduces the number of items scanned, significantly improving latency.

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

The trap here is that candidates assume high query latency is always due to insufficient read capacity or caching, rather than recognizing that inefficient query design—specifically scanning too many items—is the most common cause in time-series patterns.

Detailed technical explanation

How to think about this question

Under the hood, DynamoDB queries with a `KeyConditionExpression` on the sort key still scan all items with the same partition key in sort key order until the condition is met. Using `ScanIndexForward: false` reverses the order so the most recent items are evaluated first, and a `Limit` parameter stops the scan after retrieving the desired number of items, reducing consumed read capacity and latency. In real-world IoT scenarios, sensors generate millions of records, so without these optimizations, a query for the last hour could scan years of data.

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 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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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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 — 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 C is correct because the high query latency is most likely due to the query scanning all historical data for the sensor before applying the filter on the sort key. By default, DynamoDB queries return results in ascending order of the sort key, and without `ScanIndexForward: false` and a `Limit` parameter, the query may process many irrelevant items before reaching the last hour of data. Setting `ScanIndexForward: false` returns the most recent items first, and adding a `Limit` reduces the number of items scanned, significantly improving latency.

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.

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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