- A
Increase the Firestore quota for reads
Why wrong: Quota increase does not fix the query performance; indexing does.
- B
Create a composite index on (user_id, timestamp) for the posts collection
A composite index on (user_id, timestamp) allows Firestore to efficiently filter by user and order by timestamp.
- C
Use Cloud SQL for this query instead of Firestore
Why wrong: Switching databases is heavy; indexing is the appropriate first optimization.
- D
Denormalize the user’s recent posts into a subcollection
Why wrong: Denormalization may help but adding a composite index is a simpler first step.
Question 965 of 980
Design Scalable and Highly Available Cloud Database Solutions →mediumMultiple ChoiceObjective-mapped
PCD Practice Question: Design Scalable and Highly Available Cloud Database Solutions
This PCD practice question tests your understanding of design scalable and highly available cloud database solutions. 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 social media app uses Firestore in Native mode. The app has a feature that shows a user's recent posts. The query sorts by timestamp descending and limits to 10 results. As the user base grows, the queries become slow. Which optimization should you implement FIRST?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"first"Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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, timestamp) for the posts collection
Option B is correct because the query filters by `user_id` and sorts by `timestamp` descending, which requires a composite index on `(user_id, timestamp DESC)` to avoid a full collection scan. Without this index, Firestone performs a sort in memory, which becomes slow as the dataset grows. Creating the composite index allows Firestore to serve the query directly from the index, dramatically improving performance.
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.
- ✗
Increase the Firestore quota for reads
Why it's wrong here
Quota increase does not fix the query performance; indexing does.
- ✓
Create a composite index on (user_id, timestamp) for the posts collection
Why this is correct
A composite index on (user_id, timestamp) allows Firestore to efficiently filter by user and order by timestamp.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Use Cloud SQL for this query instead of Firestore
Why it's wrong here
Switching databases is heavy; indexing is the appropriate first optimization.
- ✗
Denormalize the user’s recent posts into a subcollection
Why it's wrong here
Denormalization may help but adding a composite index is a simpler first step.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume Firestore automatically handles all sorting or that increasing quotas or denormalizing data will fix performance, when in fact the missing composite index is the single most impactful first optimization for this query pattern.
Detailed technical explanation
How to think about this question
Firestore in Native mode automatically creates single-field indexes but requires explicit composite indexes for queries that combine equality filters (e.g., `user_id == X`) with an order on a different field (e.g., `timestamp DESC`). Without the composite index, Firestore must retrieve all documents matching the equality filter, sort them in memory, and then apply the limit, which degrades linearly with the number of matching documents. In a real-world scenario with millions of users, this in-memory sort can cause query latency to exceed 10 seconds, while a composite index reduces it to milliseconds.
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 PCD question test?
Design Scalable and Highly Available Cloud Database Solutions — This question tests Design Scalable and Highly Available Cloud Database Solutions — 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, timestamp) for the posts collection — Option B is correct because the query filters by `user_id` and sorts by `timestamp` descending, which requires a composite index on `(user_id, timestamp DESC)` to avoid a full collection scan. Without this index, Firestone performs a sort in memory, which becomes slow as the dataset grows. Creating the composite index allows Firestore to serve the query directly from the index, dramatically improving performance.
What should I do if I get this PCD question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jul 4, 2026
This PCD practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCD exam.
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