- A
Add a composite index on (author_id, created_at DESC).
This index directly supports the query, allowing an index range scan and limit.
- B
Partition the table by author_id using range partitioning.
Why wrong: Partitioning can help but ordering by created_at across partitions still requires scanning multiple partitions.
- C
Increase the query cache size in Cloud SQL.
Why wrong: Caching is a temporary fix and not a schema design change.
- D
Migrate the posts table to Cloud Spanner and use interleaved indexes.
Why wrong: This is a database migration, not a schema change within Cloud SQL.
Question 607 of 1,000
PCDE Composite index Practice Question
This PCDE practice question tests your understanding of design and implement database schemas. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: composite index. 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 startup uses Cloud SQL (MySQL) for a blogging platform. The schema has a table 'posts' with columns: post_id (auto-increment PK), title, content, author_id, created_at. The application frequently runs a query to display the latest 10 posts from a specific author: SELECT * FROM posts WHERE author_id = ? ORDER BY created_at DESC LIMIT 10. This query is slow when an author has thousands of posts. The team wants to optimize this query without changing the application code. What schema change will be most effective?
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
✓
Add a composite index on (author_id, created_at DESC).
Option A is correct. A composite index on (author_id, created_at DESC) allows the database to efficiently locate posts for a given author sorted by creation date without scanning all rows. Option B (partitioning by author_id) does not directly help because ordering across partitions would still require sorting or scanning all partitions. Option C (increasing query cache) is not a schema change and may not help if the query is not cached or the data changes frequently. Option D (migrating to Spanner) is a drastic change and not necessary; a well-designed index in Cloud SQL can solve the issue.
Key principle: Composite index
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Add a composite index on (author_id, created_at DESC).
Why this is correct
This index directly supports the query, allowing an index range scan and limit.
Related concept
Composite index
- ✗
Partition the table by author_id using range partitioning.
Why it's wrong here
Partitioning can help but ordering by created_at across partitions still requires scanning multiple partitions.
- ✗
Increase the query cache size in Cloud SQL.
Why it's wrong here
Caching is a temporary fix and not a schema design change.
- ✗
Migrate the posts table to Cloud Spanner and use interleaved indexes.
Why it's wrong here
This is a database migration, not a schema change within Cloud SQL.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Watch out for the option letter mix-up. Partitioning might seem useful but does not optimally support ORDER BY and LIMIT across partitions; a composite index is more effective.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- Composite index
- Index sort order
- Covering index
- Query optimization
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
Composite index
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. Composite index 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.
Review composite index, then practise related PCDE questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this PCDE question test?
Design and implement database schemas — This question tests Design and implement database schemas — Composite index.
What is the correct answer to this question?
The correct answer is: Add a composite index on (author_id, created_at DESC). — Option A is correct. A composite index on (author_id, created_at DESC) allows the database to efficiently locate posts for a given author sorted by creation date without scanning all rows. Option B (partitioning by author_id) does not directly help because ordering across partitions would still require sorting or scanning all partitions. Option C (increasing query cache) is not a schema change and may not help if the query is not cached or the data changes frequently. Option D (migrating to Spanner) is a drastic change and not necessary; a well-designed index in Cloud SQL can solve the issue.
What should I do if I get this PCDE question wrong?
Review composite index, then practise related PCDE questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Composite index
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Last reviewed: Jun 24, 2026
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