- A
Implement an application-level cache using ElastiCache
Why wrong: Caching reduces load but does not fix the inefficient query.
- B
Create a composite index on (user_id, login_time)
This index covers both the WHERE and ORDER BY clauses.
- C
Partition the table by user_id
Why wrong: Partitioning may reduce scan scope but still requires sorting; an index is more efficient.
- D
Upgrade to a larger instance type with more memory
Why wrong: More memory may help caching but does not eliminate the full table scan.
Question 1,217 of 1,730
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?
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
✓
Create a composite index on (user_id, login_time)
The query filters on user_id and orders by login_time, which is a classic case for a composite index. A B-Tree index on (user_id, login_time) allows MySQL to locate all rows for the given user_id via the index's leading column and then retrieve the rows in sorted order using the second column, avoiding a full table scan and a filesort operation. This directly addresses the root cause — the lack of an index to support both the WHERE and ORDER BY clauses efficiently.
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.
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
The DBS-C01 exam often tests the misconception that adding more resources (Option D) or partitioning (Option C) can substitute for proper indexing, when in fact the most efficient fix for a query with a WHERE and ORDER BY on different columns is a composite index that covers both.
Detailed technical explanation
How to think about this question
In MySQL, a composite index on (user_id, login_time) creates a B-Tree where rows are sorted first by user_id and then by login_time within each user_id. When the query uses ORDER BY login_time DESC, MySQL can traverse the index in reverse order to retrieve the top 10 rows without a filesort. The EXPLAIN plan would change from 'Using where; Using index' to 'Using where; Using index' with no 'Using filesort', confirming the index covers both the filter and sort operations.
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 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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this DBS-C01 question test?
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) — The query filters on user_id and orders by login_time, which is a classic case for a composite index. A B-Tree index on (user_id, login_time) allows MySQL to locate all rows for the given user_id via the index's leading column and then retrieve the rows in sorted order using the second column, avoiding a full table scan and a filesort operation. This directly addresses the root cause — the lack of an index to support both the WHERE and ORDER BY clauses efficiently.
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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Last reviewed: Jul 4, 2026
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