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 healthcare company stores patient records in Amazon DynamoDB. Each record includes a 'patient_id' (partition key) and 'visit_date' (sort key). The company needs to run ad-hoc queries to find all patients seen by a specific doctor within a date range. Which design approach minimizes cost and latency for this query pattern?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a global secondary index (GSI) with doctor_id as partition key and visit_date as sort key.
Option C is correct because a Global Secondary Index (GSI) with doctor_id as the partition key and visit_date as the sort key allows efficient key-based queries for all patients seen by a specific doctor within a date range. This avoids full table scans and filter operations, minimizing both cost (read capacity units) and latency. DynamoDB can directly retrieve the indexed items without scanning the base table.
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.
✗
Query the base table using a filter expression on doctor_id.
Create a local secondary index (LSI) with doctor_id as sort key.
Why it's wrong here
LSI shares the same partition key (patient_id), so cannot query by doctor alone.
✓
Create a global secondary index (GSI) with doctor_id as partition key and visit_date as sort key.
Why this is correct
GSI enables efficient query by doctor and date range.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use a Scan operation with a filter expression for doctor_id and visit_date.
Why it's wrong here
Scan reads all items, which is costly and slow.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse LSIs and GSIs, assuming an LSI can support queries on any attribute, but LSIs are restricted to the same partition key as the base table, making them unsuitable for querying by doctor_id across all patients.
Detailed technical explanation
How to think about this question
A GSI in DynamoDB maintains a separate copy of the data with a different partition key and sort key, allowing efficient querying on non-key attributes. The GSI's write capacity is shared with the base table, but reads are independent; for this pattern, the GSI enables a Query operation that retrieves only matching items by doctor_id and visit_date, using the index's sort key to filter by date range without scanning. In real-world scenarios, this design is critical for compliance with healthcare data access patterns, where ad-hoc queries must be fast and cost-effective.
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 global secondary index (GSI) with doctor_id as partition key and visit_date as sort key. — Option C is correct because a Global Secondary Index (GSI) with doctor_id as the partition key and visit_date as the sort key allows efficient key-based queries for all patients seen by a specific doctor within a date range. This avoids full table scans and filter operations, minimizing both cost (read capacity units) and latency. DynamoDB can directly retrieve the indexed items without scanning the base table.
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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