DBS-C01 Workload-Specific Database Design Practice Question
A company uses Amazon DynamoDB for a real-time analytics platform. The table has a partition key of 'customer_id' and a sort key of 'event_timestamp'. The table receives 50,000 write requests per second, evenly distributed across 10,000 customers. The application frequently queries the last 10 events for a given customer. The company notices that some queries are throttled during peak hours. The table's write capacity is set to 50,000 WCUs, and read capacity to 10,000 RCUs. The throttled queries are read requests. What is the most likely cause of the throttling, and what should be done to resolve it?
⚠ Common exam trap
The DBS-C01 exam often tests the misconception that throttling always requires increasing capacity, when in fact optimizing the access pattern with Query and Limit can resolve the issue without additional cost.
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
✓
Optimize the query by using Query with KeyConditionExpression on the sort key and Limit=10.
The throttling occurs because the application uses Scan or an inefficient query pattern that consumes excessive read capacity. Using Query with KeyConditionExpression on the sort key and Limit=10 retrieves only the last 10 events per customer efficiently, reducing read consumption and avoiding throttling without increasing RCUs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the write capacity units to handle the write load.
Why it's wrong here
Write capacity is not the issue; the throttling is on reads.
- ✗
Increase the read capacity units to 20,000 RCUs.
Why it's wrong here
Increasing capacity may reduce throttling but does not fix the underlying inefficient query pattern.
- ✓
Optimize the query by using Query with KeyConditionExpression on the sort key and Limit=10.
Why this is correct
This ensures the query reads only the necessary items, reducing RCU consumption.
- ✗
Add a global secondary index with the same keys to distribute read load.
Why it's wrong here
A GSI would not necessarily improve query efficiency; the query still needs to be optimized.
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