The answer is to create a local secondary index (LSI) with user_id as the partition key and timestamp as the sort key. This design is correct because an LSI shares the same partition key as the base table but allows a different sort key, enabling efficient retrieval of the most recent posts per user without scanning the entire table. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your understanding of when to use an LSI versus a GSI—a common trap is assuming a GSI is always better, but an LSI avoids extra write costs and provisioned throughput for queries that stay within a single partition. The key insight is that since the base table already uses user_id as the partition key, an LSI provides a sorted view of timestamps per user at no additional storage overhead for the partition key. Memory tip: LSI stands for “Local Same Index”—same partition key, different sort key, perfect for per-user time-based queries.
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 application stores user posts in a DynamoDB table with a partition key of user_id and a sort key of timestamp. The most frequent query is to retrieve the 10 most recent posts for a given user. Which secondary index design would optimize this query?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create an LSI with user_id as the partition key and timestamp as the sort key.
Option B is correct because a local secondary index (LSI) with the same partition key but a different sort key allows efficient retrieval of recent posts per user. Option A is wrong because a global secondary index (GSI) would require additional provisioned throughput and is unnecessary. Option C is wrong because using the sort key directly without an index still requires a scan. Option D is wrong because Elasticsearch adds operational complexity and cost for this simple query pattern.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Create a GSI with user_id as the partition key and timestamp as the sort key.
Why it's wrong here
A GSI would work but is overkill and adds cost compared to an LSI.
✗
Use DynamoDB Streams to populate an Amazon Elasticsearch cluster for search queries.
Why it's wrong here
This adds unnecessary complexity; the LSI is simpler and sufficient.
✓
Create an LSI with user_id as the partition key and timestamp as the sort key.
Why this is correct
An LSI uses the same partition key and a different sort key, enabling efficient range queries.
Related concept
Static NAT maps one inside address to one outside address.
✗
Query the base table using the sort key timestamp with a limit of 10.
Why it's wrong here
Without an LSI, the query would require a full scan of all items for the user.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DBS-C01 NAT questions on configuration and troubleshooting.
Workload-Specific Database Design — This question tests Workload-Specific Database Design — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Create an LSI with user_id as the partition key and timestamp as the sort key. — Option B is correct because a local secondary index (LSI) with the same partition key but a different sort key allows efficient retrieval of recent posts per user. Option A is wrong because a global secondary index (GSI) would require additional provisioned throughput and is unnecessary. Option C is wrong because using the sort key directly without an index still requires a scan. Option D is wrong because Elasticsearch adds operational complexity and cost for this simple query pattern.
What should I do if I get this DBS-C01 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related DBS-C01 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Question Discussion
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