The answer is to use conversation ID as the partition key and timestamp as the sort key, then query with ScanIndexForward=false and Limit=50. This design is correct because DynamoDB’s sort key natively orders items, so setting ScanIndexForward to false retrieves messages in reverse chronological order, while the Limit parameter restricts results to exactly the 50 most recent messages, avoiding any full table scans or secondary indexes. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your understanding of single-table design and efficient query patterns for time-series data, a common scenario in real-time chat applications. A frequent trap is overcomplicating the solution with Global Secondary Indexes or streams, when a simple composite primary key with the sort key handles the access pattern directly. Memory tip: think “PK for the chat, SK for the clock, flip the scan to stop the clock at 50.”
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 building a real-time chat application using Amazon DynamoDB. Each message has a conversation ID, timestamp, sender, and content. The primary access pattern is to retrieve the most recent 50 messages for a given conversation, ordered by timestamp. Which table design minimizes cost and latency?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use conversation ID as partition key and timestamp as sort key; query with ScanIndexForward=false and Limit=50.
Option C is correct because using conversation ID as the partition key and timestamp as the sort key allows a single Query operation with ScanIndexForward=false to retrieve items in reverse chronological order, and Limit=50 ensures only the most recent 50 messages are returned. This design minimizes cost by avoiding full table scans or additional indexes, and minimizes latency by leveraging DynamoDB's native sort key ordering without needing external caching or streams.
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 a composite primary key of conversation ID and message ID, and enable DynamoDB Streams to process messages.
Why it's wrong here
Message ID does not preserve order; streams are not needed.
✗
Use DynamoDB Accelerator (DAX) to cache the most recent messages.
Why it's wrong here
DAX is unnecessary and adds cost; the query is already efficient.
✓
Use conversation ID as partition key and timestamp as sort key; query with ScanIndexForward=false and Limit=50.
Why this is correct
Directly supports the access pattern without additional indexes.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use conversation ID as partition key and a GSI on timestamp.
Why it's wrong here
GSI adds unnecessary cost; the sort key can be used directly.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may over-engineer the solution by adding unnecessary components like DAX or GSIs, when DynamoDB's native sort key and query parameters directly solve the access pattern with minimal cost and latency.
Detailed technical explanation
How to think about this question
DynamoDB stores items within a partition key group sorted by sort key in ascending order by default. Setting ScanIndexForward=false reverses this order, so the most recent messages (highest timestamp) are returned first. The Limit parameter stops the query after retrieving 50 items, ensuring consistent performance regardless of total message count. This design also avoids hot partitions because conversation IDs distribute write load evenly, and the sort key enables efficient range queries without scanning.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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 conversation ID as partition key and timestamp as sort key; query with ScanIndexForward=false and Limit=50. — Option C is correct because using conversation ID as the partition key and timestamp as the sort key allows a single Query operation with ScanIndexForward=false to retrieve items in reverse chronological order, and Limit=50 ensures only the most recent 50 messages are returned. This design minimizes cost by avoiding full table scans or additional indexes, and minimizes latency by leveraging DynamoDB's native sort key ordering without needing external caching or streams.
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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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