The answer is to create a composite index on the attributes in a single collection. This design is correct because Firestore composite indexes for multiple attributes allow the database to serve queries filtering by age, city, and last_login without performing a costly collection scan, ensuring efficient query performance. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of index-based query optimization versus denormalization or subcollection patterns; a common trap is assuming that denormalized arrays or separate subcollections simplify multi-attribute filtering, but they actually increase complexity and read costs. Remember the memory tip: “One collection, one composite index—no scans, no subcollection tax.”
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 Firestore application stores user profiles that must be queried by any of multiple attributes (age, city, last_login). What is the best schema design to support these queries efficiently?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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 the attributes in a single collection
Option A is correct: a composite index on the attributes allows Firestore to serve queries without collection scans. Option B (denormalized arrays) is inefficient for filtering. Option C (subcollections) adds complexity and may require more reads. Option D (separate documents per attribute) is not practical.
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 attributes in an array field and query with array-contains
Why it's wrong here
Array-contains only checks for exact values, not range or comparison queries.
✓
Create a composite index on the attributes in a single collection
Why this is correct
Composite indexes enable efficient multi-attribute queries in Firestore.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use subcollections per attribute value
Why it's wrong here
Subcollections would require multiple collection group queries and complex logic.
✗
Create separate documents for each attribute value
Why it's wrong here
This would require merging results manually and increase complexity.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 PCDE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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: Create a composite index on the attributes in a single collection — Option A is correct: a composite index on the attributes allows Firestore to serve queries without collection scans. Option B (denormalized arrays) is inefficient for filtering. Option C (subcollections) adds complexity and may require more reads. Option D (separate documents per attribute) is not practical.
What should I do if I get this PCDE question wrong?
Identify which PCDE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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