This DBS-C01 practice question tests your understanding of workload-specific database design. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 database administrator is troubleshooting a slow-performing query on an Amazon RDS for MySQL instance. The slow query log shows the above entry. Based on the exhibit, which index would most improve the query performance?
Index on `created_at` only.
Why wrong: Does not help filtering on status.
Index on `status` only.
Why wrong: Helps filtering but not sorting; still may need file sort.
Full-text index on `status` and `created_at`.
Why wrong: Full-text indexes are for text search, not this query.
Composite index on (`status`, `created_at`).
Covers both filter and sort, avoiding a full table scan.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Composite index on (`status`, `created_at`).
The query filters on `status` and then sorts or filters on `created_at`. A composite index on (`status`, `created_at`) allows MySQL to use the index for both the equality condition on `status` and the range or sort on `created_at`, avoiding a filesort and reducing row scans. This is the most efficient index for this query pattern.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Index on `created_at` only.
Why it's wrong here
Does not help filtering on status.
Index on `status` only.
Why it's wrong here
Helps filtering but not sorting; still may need file sort.
Full-text index on `status` and `created_at`.
Why it's wrong here
Full-text indexes are for text search, not this query.
Composite index on (`status`, `created_at`).
Why this is correct
Covers both filter and sort, avoiding a full table scan.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates often pick a single-column index on `status` (Option B) thinking it will help the filter, but they overlook the need to also optimize the sort or range on `created_at`, which requires a composite index to avoid a filesort.
Detailed technical explanation
MySQL's InnoDB storage engine uses B+ tree indexes where the leftmost prefix rule applies: a composite index on (status, created_at) can satisfy queries filtering on status alone or on both columns, but not on created_at alone. The optimizer will use the index for the WHERE clause on status and then for the ORDER BY or range on created_at, eliminating the need for a temporary table or filesort. In practice, this is a classic case where a covering index can also be considered if the SELECT list is small, but the composite index is the minimal fix.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..
The correct answer is: Composite index on (`status`, `created_at`). — The query filters on `status` and then sorts or filters on `created_at`. A composite index on (`status`, `created_at`) allows MySQL to use the index for both the equality condition on `status` and the range or sort on `created_at`, avoiding a filesort and reducing row scans. This is the most efficient index for this query pattern.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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