The answer is to use a separate document per game with listeners filtering by game ID, and to store an aggregate score summary document per game. These two design decisions minimize reads and reduce costs because the first ensures each client reads only the single document relevant to their game, avoiding unnecessary document scans, while the second batches frequent score updates into one document write, so all listeners read that single, compact document instead of multiple sub-documents. On the Google Professional Cloud Database Engineer exam, this tests your understanding of Firestore’s real-time listener cost model, where every document snapshot delivered counts as a read; a common trap is choosing a collection group query, which would listen to many documents and spike reads. Remember the memory tip: “One game, one doc, one summary—keep reads low and costs no glum.”
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 uses Firestore to power a live sports score app. Scores are updated frequently, and many clients listen to real-time updates on specific games. Which two design decisions will minimize the number of reads and reduce costs? (Choose two.)
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Store an aggregate score summary document per game and listen to it
Options B and C are correct. B: using a separate document per game and having clients listen only to the game they're interested in minimizes reads because each client only reads one document. C: storing an aggregate score summary document per game reduces the number of document updates and reads because changes are batched into a single document write, and listeners read that one document. Option A (collection group query) would listen to many documents, increasing reads. Option D (subcollection of periods) increases read complexity. Option E (single document for all games) would cause document contention and all clients reading the same large document.
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 a collection group query to listen to all games at once
Why it's wrong here
Listens to many documents, increasing reads and cost.
✓
Store an aggregate score summary document per game and listen to it
Why this is correct
Reduces write operations and read frequency; clients get updates from a single summary document.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✓
Use a separate document per game and listeners filter by game ID
Why this is correct
Each client listens only to the relevant document, minimizing reads.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use a single document for all games with nested fields
Why it's wrong here
Causes contention and all clients reading a large document; not scalable.
✗
Use a subcollection of periods (quarters) to spread writes
Why it's wrong here
Increases number of documents and reads without cost benefit.
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 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
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: Store an aggregate score summary document per game and listen to it — Options B and C are correct. B: using a separate document per game and having clients listen only to the game they're interested in minimizes reads because each client only reads one document. C: storing an aggregate score summary document per game reduces the number of document updates and reads because changes are batched into a single document write, and listeners read that one document. Option A (collection group query) would listen to many documents, increasing reads. Option D (subcollection of periods) increases read complexity. Option E (single document for all games) would cause document contention and all clients reading the same large document.
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: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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