The answer is to create a local secondary index on user_id and post_timestamp. This design pattern improves query performance and reduces cost because a Local Secondary Index (LSI) shares the same partition key as the base table—user_id—while allowing a different sort key, here post_timestamp. DynamoDB can then efficiently fetch the five most recent posts for a given user by querying the index with a descending sort order and a limit of five, avoiding a full table scan and minimizing consumed read capacity. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of when to use an LSI versus a Global Secondary Index (GSI); a common trap is choosing a GSI, which would require eventual consistency and additional write capacity. Remember that an LSI provides strongly consistent reads and shares the base table’s throughput, making it ideal for queries that always filter on the same partition key. Memory tip: LSI = “Local Same Index”—same hash key, different sort key for fast, local lookups.
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 social media startup stores user posts in Amazon DynamoDB with a partition key of user_id and sort key of post_timestamp. The application frequently queries the five most recent posts for a given user. Which design pattern improves query performance and reduces cost?
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 user_id and post_timestamp
Option D is correct because a Local Secondary Index (LSI) on user_id (hash key) and post_timestamp (range key) allows DynamoDB to efficiently query the five most recent posts for a given user without scanning the entire table. Since the LSI shares the same partition key as the base table, it provides strongly consistent reads and avoids the overhead of a separate index, reducing both latency and consumed read capacity.
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) for the table
Why it's wrong here
DAX improves latency but not query efficiency for this pattern.
✗
Increase the read capacity units on the table
Why it's wrong here
Does not improve query efficiency, just increases cost.
✗
Use a global secondary index on post_timestamp
Why it's wrong here
GSI on timestamp alone does not filter by user_id.
✓
Create a local secondary index on user_id and post_timestamp
Why this is correct
LSI allows efficient query on user_id with sorted results.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose a GSI on post_timestamp (Option C) thinking it will help with sorting, but without user_id as the partition key, the GSI cannot efficiently scope the query to a single user, leading to full scans and higher costs.
Detailed technical explanation
How to think about this question
A Local Secondary Index (LSI) maintains the same partition key (user_id) as the base table but allows a different sort key (post_timestamp). DynamoDB stores LSI data in the same partition as the base table, enabling efficient range queries with the Query API using ScanIndexForward=false and Limit=5 to retrieve the most recent posts with minimal read capacity. This design leverages DynamoDB's internal sorting and avoids the eventual consistency trade-offs of a GSI, which is critical for real-time social media feeds.
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: Create a local secondary index on user_id and post_timestamp — Option D is correct because a Local Secondary Index (LSI) on user_id (hash key) and post_timestamp (range key) allows DynamoDB to efficiently query the five most recent posts for a given user without scanning the entire table. Since the LSI shares the same partition key as the base table, it provides strongly consistent reads and avoids the overhead of a separate index, reducing both latency and consumed read capacity.
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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Question Discussion
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