- A
Store all posts in a top-level collection and query for posts where user ID is in the list of followed users, ordered by timestamp.
Why wrong: Firestore cannot perform an 'IN' query with an order by on a different field efficiently.
- B
Store a feed subcollection under each user document containing references to posts from followed users.
This allows direct query on the feed subcollection ordered by timestamp.
- C
Store all user posts in an array within a single document and use array-contains queries.
Why wrong: Single document size limit is 1 MiB; not scalable.
- D
Store a 'follows' collection with documents containing follower and followed user IDs; then query posts for each followed user.
Why wrong: Requires N+1 queries and client-side merging, which is inefficient.
Quick Answer
The answer is to store a feed subcollection under each user document containing pre-computed references to posts from followed users. This design is correct because it leverages a pre-computed subcollection, meaning the feed data is written once when a followed user posts and read directly as a single indexed query ordered by timestamp, avoiding expensive collection-group scans or multiple individual lookups per followed user. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of denormalization and write amplification trade-offs in Firestore; a common trap is choosing a collection-group query on a “posts” collection, which becomes costly and slow as follow relationships scale. Remember the memory tip: “Write once per post, read once per feed” — pre-computation shifts the cost to writes, which are far less frequent than reads in a social feed, ensuring both cost-effectiveness and performance at scale.
PCDE Design and implement database schemas Practice Question
This PCDE practice question tests your understanding of design and implement database schemas. 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 is designing a Cloud Firestore schema for a social media application. Users can follow other users, and the application needs to display a feed of posts from followed users ordered by timestamp. Which schema design is most cost-effective and performant for querying the feed?
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
Store a feed subcollection under each user document containing references to posts from followed users.
Option B is correct because it uses a feed subcollection under each user document to store pre-computed references to posts from followed users. This design avoids expensive collection-group queries or multiple individual queries per followed user, ensuring that fetching the feed is a single, indexed read operation ordered by timestamp, which is both cost-effective and performant at scale.
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.
- ✗
Store all posts in a top-level collection and query for posts where user ID is in the list of followed users, ordered by timestamp.
Why it's wrong here
Firestore cannot perform an 'IN' query with an order by on a different field efficiently.
- ✓
Store a feed subcollection under each user document containing references to posts from followed users.
Why this is correct
This allows direct query on the feed subcollection ordered by timestamp.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Store all user posts in an array within a single document and use array-contains queries.
Why it's wrong here
Single document size limit is 1 MiB; not scalable.
- ✗
Store a 'follows' collection with documents containing follower and followed user IDs; then query posts for each followed user.
Why it's wrong here
Requires N+1 queries and client-side merging, which is inefficient.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose Option A, thinking a single top-level query with an 'in' filter is simpler, but they overlook Firestore's 10-value limit on 'in' queries and the resulting need for multiple queries, which destroys both performance and cost predictability at scale.
Detailed technical explanation
How to think about this question
Under the hood, Cloud Firestore indexes each subcollection independently, so a feed subcollection with a composite index on timestamp and a reference field allows a single 'orderBy' query with 'startAfter' for cursor-based pagination. In a real-world scenario, this fan-out write pattern is similar to how Twitter's early architecture used Redis lists, but Firestore's managed replication ensures eventual consistency for writes while reads remain strongly consistent within a single document's subcollection.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
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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 — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Store a feed subcollection under each user document containing references to posts from followed users. — Option B is correct because it uses a feed subcollection under each user document to store pre-computed references to posts from followed users. This design avoids expensive collection-group queries or multiple individual queries per followed user, ensuring that fetching the feed is a single, indexed read operation ordered by timestamp, which is both cost-effective and performant at scale.
What should I do if I get this PCDE 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: Jun 11, 2026
This PCDE 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 PCDE exam.
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