- A
Use arrays in the chat room document to store message IDs.
Why wrong: Arrays have size limits and are not suitable for large numbers of messages.
- B
Use a composite index on chat room ID and timestamp.
A composite index is required for querying messages efficiently.
- C
Store all messages in a single top-level collection with a field for chat room ID.
Why wrong: A single large collection can be less efficient for filtering on chat room and timestamp.
- D
Use a separate top-level collection for each chat room.
Why wrong: This results in many collections, which is not recommended due to collection limits and management overhead.
- E
Store messages as documents in a subcollection under each chat room document.
Subcollections scale independently and allow efficient queries.
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 startup is using Firestore in Native mode for a real-time chat application. They want to design the schema for chat rooms and messages. Which TWO design patterns are recommended? (Choose two.)
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
Use a composite index on chat room ID and timestamp.
Option B is correct because a composite index on chat room ID and timestamp is essential for efficiently querying messages in order within a specific chat room. Firestore requires composite indexes for queries that combine equality filters on one field (chat room ID) with an order on another (timestamp). Option E is correct because storing messages as documents in a subcollection under each chat room document is the recommended pattern for Firestore, as it allows scalable, independent message collections per room without hitting the 1 MiB document size limit.
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.
- ✗
Use arrays in the chat room document to store message IDs.
Why it's wrong here
Arrays have size limits and are not suitable for large numbers of messages.
- ✓
Use a composite index on chat room ID and timestamp.
Why this is correct
A composite index is required for querying messages efficiently.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Store all messages in a single top-level collection with a field for chat room ID.
Why it's wrong here
A single large collection can be less efficient for filtering on chat room and timestamp.
- ✗
Use a separate top-level collection for each chat room.
Why it's wrong here
This results in many collections, which is not recommended due to collection limits and management overhead.
- ✓
Store messages as documents in a subcollection under each chat room document.
Why this is correct
Subcollections scale independently and allow efficient queries.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Firestore exams often test the misconception that arrays are suitable for storing related data, but Firestore arrays lack the indexing and scalability needed for relational-like references, leading candidates to incorrectly choose Option A.
Detailed technical explanation
How to think about this question
Firestore subcollections are stored independently from their parent document, allowing messages to scale without affecting the parent document's read/write performance. Composite indexes in Firestore are automatically created for simple queries, but custom composite indexes (e.g., on chatRoomId and timestamp) must be defined in the console or via gcloud CLI to support range queries with ordering. A real-world scenario: a chat app with millions of messages per room would fail if using arrays (1 MiB limit) or a single collection (index write overhead), but subcollections with composite indexes handle this efficiently by distributing writes across multiple documents.
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.
- →
Design and implement database schemas — study guide chapter
Learn the concepts, then practise the questions
- →
Design and implement database schemas practice questions
Targeted practice on this topic area only
- →
All PCDE questions
1,000 questions across all exam domains
- →
Google Professional Cloud Database Engineer study guide
Full concept coverage aligned to exam objectives
- →
PCDE practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related PCDE practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Building and Implementing CI/CD Pipelines for a Service practice questions
Practise PCDE questions linked to Building and Implementing CI/CD Pipelines for a Service.
Bootstrapping a Google Cloud Organisation for DevOps practice questions
Practise PCDE questions linked to Bootstrapping a Google Cloud Organisation for DevOps.
Applying Site Reliability Engineering Practices to a Service practice questions
Practise PCDE questions linked to Applying Site Reliability Engineering Practices to a Service.
Implementing Service Monitoring Strategies practice questions
Practise PCDE questions linked to Implementing Service Monitoring Strategies.
Optimising Service Performance practice questions
Practise PCDE questions linked to Optimising Service Performance.
Plan and manage database infrastructure practice questions
Practise PCDE questions linked to Plan and manage database infrastructure.
Define data structures and implement SQL for Business Intelligence practice questions
Practise PCDE questions linked to Define data structures and implement SQL for Business Intelligence.
Design and implement database schemas practice questions
Practise PCDE questions linked to Design and implement database schemas.
Monitor and optimize database performance practice questions
Practise PCDE questions linked to Monitor and optimize database performance.
PCDE fundamentals practice questions
Practise PCDE questions linked to PCDE fundamentals.
PCDE scenario practice questions
Practise PCDE questions linked to PCDE scenario.
PCDE troubleshooting practice questions
Practise PCDE questions linked to PCDE troubleshooting.
Practice this exam
Start a free PCDE practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
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: Use a composite index on chat room ID and timestamp. — Option B is correct because a composite index on chat room ID and timestamp is essential for efficiently querying messages in order within a specific chat room. Firestore requires composite indexes for queries that combine equality filters on one field (chat room ID) with an order on another (timestamp). Option E is correct because storing messages as documents in a subcollection under each chat room document is the recommended pattern for Firestore, as it allows scalable, independent message collections per room without hitting the 1 MiB document size limit.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Keep practising
More PCDE practice questions
- A company stores sensor data in BigQuery. They have a table 'sensor_readings' with columns: sensor_id, reading_time, val…
- Which THREE are valid considerations when designing BigQuery tables for BI reporting?
- A company uses Cloud Build to build Docker images. They want to cache intermediate layers to speed up subsequent builds.…
- A company is adopting GitOps for their GKE clusters using Config Sync. They need to meet the following requirements: (1)…
- A team is migrating an on-premises PostgreSQL database to Cloud SQL for PostgreSQL. The existing schema uses a large num…
- A DevOps engineer is setting up Docker credential helper for Artifact Registry on a Cloud Build worker. They want the bu…
Last reviewed: Jul 4, 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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.