- A
Use BatchGetItem with the sensor ID and multiple timestamps
Why wrong: BatchGetItem requires known sort keys and cannot find the latest without prior knowledge.
- B
Use Scan with FilterExpression on sensor ID
Why wrong: Scan reads all items and is inefficient.
- C
Use GetItem with the sensor ID and the current timestamp
Why wrong: The exact timestamp is unknown; this would require an additional query to find the latest.
- D
Use Query with KeyConditionExpression on sensor ID, ScanIndexForward=false, Limit=1
This retrieves the most recent item for that sensor efficiently.
Question 1,336 of 1,730
Retrieving the Latest Item per Partition Key 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. 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 to store IoT sensor data. Each sensor sends data every second, and the application needs to query the latest reading from each sensor. The sensor ID is the partition key, and the timestamp is the sort key. The table has millions of sensors. Which query pattern is most efficient to get the latest reading for a specific sensor?
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 Query with KeyConditionExpression on sensor ID, ScanIndexForward=false, Limit=1
Option D is correct because Query with ScanIndexForward=false and Limit=1 retrieves only the most recent item for a given partition key (sensor ID) by reading items in descending sort key (timestamp) order and stopping after one item. This is the most efficient pattern as it uses the primary key index directly, avoids scanning, and minimizes read capacity 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.
- ✗
Use BatchGetItem with the sensor ID and multiple timestamps
Why it's wrong here
BatchGetItem requires known sort keys and cannot find the latest without prior knowledge.
- ✗
Use Scan with FilterExpression on sensor ID
Why it's wrong here
Scan reads all items and is inefficient.
- ✗
Use GetItem with the sensor ID and the current timestamp
Why it's wrong here
The exact timestamp is unknown; this would require an additional query to find the latest.
- ✓
Use Query with KeyConditionExpression on sensor ID, ScanIndexForward=false, Limit=1
Why this is correct
This retrieves the most recent item for that sensor efficiently.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The DBS-C01 exam often tests the misconception that GetItem can be used with a partial key or that Scan with a filter is acceptable for single-item retrieval, but the trap here is that candidates overlook the efficiency of using Query with sort key ordering and limit to fetch the most recent item without scanning or guessing timestamps.
Detailed technical explanation
How to think about this question
DynamoDB stores items with the same partition key physically adjacent, sorted by sort key in ascending order. By setting ScanIndexForward=false, the query reads items in descending order, and Limit=1 stops after the first item, which is the latest timestamp. This pattern consumes only 1 read capacity unit (for items up to 4 KB) and leverages the local secondary index or table index efficiently, making it ideal for time-series data where the latest record is frequently needed.
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
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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: Use Query with KeyConditionExpression on sensor ID, ScanIndexForward=false, Limit=1 — Option D is correct because Query with ScanIndexForward=false and Limit=1 retrieves only the most recent item for a given partition key (sensor ID) by reading items in descending sort key (timestamp) order and stopping after one item. This is the most efficient pattern as it uses the primary key index directly, avoids scanning, and minimizes read capacity consumption.
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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Last reviewed: Jul 4, 2026
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