The correct answer is to create a composite index on (user_id, login_time). This design change directly addresses the root cause of the full table scan by enabling the database engine to locate rows for a specific user via the leading column and then retrieve them in the required sorted order using the second column, all without scanning the entire 50-million-row table. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of index key order and how composite indexes can satisfy both a WHERE filter and an ORDER BY clause in a single index seek, a common optimization pattern for high-volume transactional workloads. A frequent trap is assuming that partitioning or scaling up hardware alone will fix performance, but those solutions do not eliminate the need for an efficient access path. Remember the memory tip: “Filter first, then sort” — always place the equality column before the sort column in a composite index to avoid a filesort.
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 company uses Amazon RDS for MySQL with Multi-AZ and read replicas. The database has a table storing user sessions with 50 million rows. The application team reports that queries using 'SELECT * FROM sessions WHERE user_id = ? ORDER BY login_time DESC LIMIT 10' are slow. The EXPLAIN plan shows a full table scan. Which design change would BEST improve query performance?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a composite index on (user_id, login_time)
Option A is correct because a composite index on (user_id, login_time) allows the database to find rows by user and order by time without scanning. Option B is wrong because increasing instance size does not address the missing index. Option C is wrong because caching may help but does not fix the root cause. Option D is wrong because partitioning by user_id may help, but a composite index is more effective for this specific query.
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.
✗
Implement an application-level cache using ElastiCache
Why it's wrong here
Caching reduces load but does not fix the inefficient query.
✓
Create a composite index on (user_id, login_time)
Why this is correct
This index covers both the WHERE and ORDER BY clauses.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Partition the table by user_id
Why it's wrong here
Partitioning may reduce scan scope but still requires sorting; an index is more efficient.
✗
Upgrade to a larger instance type with more memory
Why it's wrong here
More memory may help caching but does not eliminate the full table scan.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
Got this wrong? Here's your next step.
Identify which DBS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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 composite index on (user_id, login_time) — Option A is correct because a composite index on (user_id, login_time) allows the database to find rows by user and order by time without scanning. Option B is wrong because increasing instance size does not address the missing index. Option C is wrong because caching may help but does not fix the root cause. Option D is wrong because partitioning by user_id may help, but a composite index is more effective for this specific query.
What should I do if I get this DBS-C01 question wrong?
Identify which DBS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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