The answer is to avoid monotonically increasing primary keys and instead use secondary indexes for non-key column queries. This is correct because a sequential primary key, such as an auto-increment integer or timestamp, creates a hot spot on a single tablet server, serializing all writes into one split and causing severe write contention. Cloud Spanner distributes data across splits based on primary key range, so a hash prefix or UUID-like key spreads writes evenly, maximizing parallelism and throughput for high-performance schema design. On the Google Professional Cloud Database Engineer exam, this principle tests your understanding of distributed database architecture and write hot-spotting, a common trap where candidates assume sequential keys are efficient. Remember the memory tip: “Don’t let your keys climb—spread them wide and hash the tide.”
PCDE Monitor and optimize database performance Practice Question
This PCDE practice question tests your understanding of monitor and optimize database performance. 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.
Which THREE are best practices for designing a Cloud Spanner schema for high performance? (Choose three.)
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
✓
Avoid using a monotonically increasing primary key as the first part of the key.
Option B is correct because a monotonically increasing primary key (e.g., an auto-increment integer or timestamp) creates a hot spot on the last tablet server, causing all writes to be serialized on a single split. Cloud Spanner distributes splits based on the primary key range; a sequential key forces all new inserts into the same split, leading to write contention and poor throughput. Using a hash prefix or a UUID-like key spreads writes evenly across splits, maximizing parallelism.
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.
✗
Spread data across multiple regions to reduce latency.
Why it's wrong here
Multi-region increases write latency.
✓
Avoid using a monotonically increasing primary key as the first part of the key.
Why this is correct
This prevents write hotspotting.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Denormalize frequently joined tables into a single table.
Why it's wrong here
Denormalization may cause data consistency issues.
✓
Use interleaved tables for tables that are always accessed together by the parent key.
Why this is correct
Interleaving reduces cross-node reads.
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 secondary indexes when querying by non-key columns.
Why this is correct
Secondary indexes allow efficient lookup.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that denormalization is always beneficial for performance, but in Cloud Spanner, interleaved tables provide efficient parent-child joins without the downsides of denormalization.
Detailed technical explanation
How to think about this question
Cloud Spanner splits data into tablets based on the primary key range, and each tablet is replicated across zones using Paxos. A monotonically increasing key causes all new rows to land in the last tablet, creating a write hotspot that cannot be automatically split until it reaches a certain size. Using a hash of the key as the first part of the primary key distributes writes across multiple tablets from the start, allowing Cloud Spanner to parallelize writes and avoid throttling.
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
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.
Monitor and optimize database performance — This question tests Monitor and optimize database performance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Avoid using a monotonically increasing primary key as the first part of the key. — Option B is correct because a monotonically increasing primary key (e.g., an auto-increment integer or timestamp) creates a hot spot on the last tablet server, causing all writes to be serialized on a single split. Cloud Spanner distributes splits based on the primary key range; a sequential key forces all new inserts into the same split, leading to write contention and poor throughput. Using a hash prefix or a UUID-like key spreads writes evenly across splits, maximizing parallelism.
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.
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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