- A
Use the timestamp of order creation as the primary key.
Why wrong: A monotonically increasing timestamp concentrates writes on a single split, causing hotspots.
- B
Use a sequential integer primary key with auto-increment.
Why wrong: Sequential integers concentrate writes on the last split, causing hotspots.
- C
Use a composite primary key starting with a hash of the order ID, followed by the order ID.
A hash prefix ensures writes are distributed across all splits, avoiding hotspots.
- D
Keep UUID as primary key but add a secondary index on a hash of the UUID.
Why wrong: Adding an index does not change the primary key write distribution; the primary key still causes hotspots.
Question 875 of 1,000
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 global e-commerce company is designing a Cloud Spanner schema for order processing. They need strong consistency across regions and high write throughput. Orders are identified by a globally unique order ID (UUID). Currently, they use the UUID as the primary key, but they observe write hotspots during peak hours. What primary key design change should they make to distribute writes more evenly?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"primary"Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
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 primary key starting with a hash of the order ID, followed by the order ID.
Option C is correct because using a composite primary key starting with a hash of the order ID distributes writes evenly across all Cloud Spanner nodes, preventing hotspots. Cloud Spanner uses the first column of the primary key to determine row distribution; a monotonically increasing UUID as the first column causes all new writes to land on the same tablet, creating a hotspot. By hashing the UUID first, writes are spread uniformly across the key space, while the order ID ensures uniqueness.
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 the timestamp of order creation as the primary key.
Why it's wrong here
A monotonically increasing timestamp concentrates writes on a single split, causing hotspots.
- ✗
Use a sequential integer primary key with auto-increment.
Why it's wrong here
Sequential integers concentrate writes on the last split, causing hotspots.
- ✓
Use a composite primary key starting with a hash of the order ID, followed by the order ID.
Why this is correct
A hash prefix ensures writes are distributed across all splits, avoiding hotspots.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Keep UUID as primary key but add a secondary index on a hash of the UUID.
Why it's wrong here
Adding an index does not change the primary key write distribution; the primary key still causes hotspots.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often think secondary indexes or adding a hash anywhere in the schema will fix distribution, but Cloud Spanner only distributes rows based on the first column of the primary key, so the hash must be the leading column.
Detailed technical explanation
How to think about this question
Cloud Spanner splits data into tablets based on the primary key's first column using range partitioning. A hash prefix ensures that writes are uniformly distributed across all tablets, avoiding the 'last-page' hotspot problem common with monotonically increasing keys. In real-world scenarios, this design is critical for achieving the high write throughput required during flash sales or peak order processing, where millions of writes per second must be evenly distributed across Spanner's global nodes.
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.
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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: Use a composite primary key starting with a hash of the order ID, followed by the order ID. — Option C is correct because using a composite primary key starting with a hash of the order ID distributes writes evenly across all Cloud Spanner nodes, preventing hotspots. Cloud Spanner uses the first column of the primary key to determine row distribution; a monotonically increasing UUID as the first column causes all new writes to land on the same tablet, creating a hotspot. By hashing the UUID first, writes are spread uniformly across the key space, while the order ID ensures uniqueness.
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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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