The answer is to use a Global Secondary Index (GSI) with a sort key that matches the reporting query pattern. This is correct because a full table scan consumes read capacity units (RCUs) for every item, even those irrelevant to the report, while a GSI allows the reporting job to query only the specific items needed, drastically reducing RCU consumption under on-demand capacity and thus lowering costs. On the AWS Certified Data Engineer Associate DEA-C01 exam, this scenario tests your understanding of DynamoDB cost optimization for reporting scans and the trade-off between scan efficiency and index storage costs. A common trap is to suggest adding more RCUs or switching to provisioned capacity, but the core concept is that querying a well-designed GSI eliminates unnecessary reads. Memory tip: “GSI for query, not for scan—filter first, pay less per plan.”
DEA-C01 Data Store Management Practice Question
This DEA-C01 practice question tests your understanding of data store management. 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 uses Amazon DynamoDB with on-demand capacity. They notice higher than expected costs due to a sudden spike in read traffic from a reporting job. The reporting job scans the entire table daily. What is the most cost-effective way to reduce costs while maintaining the same reporting output?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a Global Secondary Index (GSI) with a sort key that matches the reporting query pattern.
Option B is correct because using a Global Secondary Index (GSI) with a sort key tailored to the reporting query pattern allows the reporting job to query only the relevant items instead of scanning the entire table. This reduces the read capacity units consumed per operation, directly lowering costs under on-demand capacity, which charges per RCU consumed. The reporting output remains identical because the GSI returns the same data filtered by the query pattern.
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 caching.
Why it's wrong here
DAX adds cost and may not eliminate scans fully.
✓
Use a Global Secondary Index (GSI) with a sort key that matches the reporting query pattern.
Why this is correct
A GSI allows efficient querying instead of scanning, reducing read costs.
Related concept
Read the scenario before looking for a memorised answer.
✗
Set a TTL attribute to automatically expire old data.
Why it's wrong here
TTL removes old data but does not optimize the reporting query.
✗
Reduce the read capacity units (RCU) in the table.
Why it's wrong here
On-demand capacity cannot be manually reduced; it auto-scales.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse DAX as a general cost-saver for all read patterns, but DAX only helps with repeated, cached reads, not with unique full-table scans that read different data each time.
Detailed technical explanation
How to think about this question
Under the hood, a DynamoDB scan reads every item in the table, consuming RCUs equal to the sum of item sizes (up to 4 KB per RCU for eventually consistent reads). A GSI with a sort key that matches the reporting query pattern enables a Query operation, which only reads items matching the partition key and sort key condition, drastically reducing the number of items read. In on-demand mode, costs are directly proportional to RCU consumption, so this optimization can cut costs by orders of magnitude for large tables.
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.
Data Store Management — This question tests Data Store Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a Global Secondary Index (GSI) with a sort key that matches the reporting query pattern. — Option B is correct because using a Global Secondary Index (GSI) with a sort key tailored to the reporting query pattern allows the reporting job to query only the relevant items instead of scanning the entire table. This reduces the read capacity units consumed per operation, directly lowering costs under on-demand capacity, which charges per RCU consumed. The reporting output remains identical because the GSI returns the same data filtered by the query pattern.
What should I do if I get this DEA-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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