- A
Reverse the timestamp and use it as the primary key.
Why wrong: Reversing a timestamp can help but if timestamps are close together, may still cause hotspots.
- B
Use a UUID as the primary key to ensure randomness.
Why wrong: UUIDs distribute writes but are large and may lead to poor read locality; also, they are not sequential, which may complicate range queries.
- C
Use a composite primary key with a timestamp and a random number.
Why wrong: Adding a random number helps distribution but makes point lookups harder without also including the transaction ID.
- D
Use a composite primary key with a hash prefix of the transaction ID as the first component, followed by the transaction ID.
The hash prefix evenly distributes writes, and the transaction ID allows efficient point lookups.
Cloud Spanner Primary Key Design: Hash Prefix to Avoid Hotspots
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 financial services company uses Cloud Spanner for a ledger application. The ledger table has a primary key of 'transaction_id' which is a monotonically increasing integer. During peak hours, they observe high write latencies due to hot spots on the last tablet. They need to redesign the schema to distribute writes evenly while still allowing efficient point lookups by transaction ID. What is the best approach?
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.
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.
Quick Answer
The answer is to use a composite primary key with a hash prefix of the transaction ID as the first component, followed by the transaction ID itself. This approach directly addresses the hotspot problem by ensuring that the monotonically increasing transaction IDs are not written sequentially to the same tablet; instead, the hash prefix distributes writes evenly across all tablets in the Cloud Spanner node, while keeping the original transaction ID as the second component allows for efficient point lookups. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of how primary key design directly impacts write throughput and tablet splitting in Spanner. A common trap is choosing a UUID, which also distributes writes but increases key size and can degrade read performance, or a reverse timestamp, which may still create hotspots if timestamps are sequential. Remember the memory tip: “Hash the hot, keep the spot”—hash the hot key prefix to spread writes, but keep the original key for direct lookups.
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 with a hash prefix of the transaction ID as the first component, followed by the transaction ID.
Option D is correct because using a hash prefix of the transaction ID as the first component of a composite primary key distributes writes across multiple tablets in Cloud Spanner, avoiding hot spots on a single tablet. The transaction ID as the second component still enables efficient point lookups by transaction ID, as Spanner can use the hash prefix to locate the tablet and then scan within it for the exact row.
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.
- ✗
Reverse the timestamp and use it as the primary key.
Why it's wrong here
Reversing a timestamp can help but if timestamps are close together, may still cause hotspots.
- ✗
Use a UUID as the primary key to ensure randomness.
Why it's wrong here
UUIDs distribute writes but are large and may lead to poor read locality; also, they are not sequential, which may complicate range queries.
- ✗
Use a composite primary key with a timestamp and a random number.
Why it's wrong here
Adding a random number helps distribution but makes point lookups harder without also including the transaction ID.
- ✓
Use a composite primary key with a hash prefix of the transaction ID as the first component, followed by the transaction ID.
Why this is correct
The hash prefix evenly distributes writes, and the transaction ID allows efficient point lookups.
Clue confirmation
The clue words "best", "primary" 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
A common mistake in Google Cloud Spanner design is thinking that simply adding randomness (like a UUID or random number) solves hot spots, but this fails to consider that the primary key must still support efficient point lookups by the original identifier, which a composite key with a hash prefix achieves.
Detailed technical explanation
How to think about this question
Cloud Spanner uses range-based sharding by default, so monotonically increasing keys like transaction IDs cause all new writes to land on the last tablet (hot spot). By using a hash prefix (e.g., a 4-byte hash of the transaction ID) as the first key component, writes are distributed across tablets because the hash values are uniformly distributed. The transaction ID as the second component allows Spanner to perform a point lookup by first computing the hash prefix to locate the tablet, then using the transaction ID to find the exact row within that tablet's sorted range.
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
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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 with a hash prefix of the transaction ID as the first component, followed by the transaction ID. — Option D is correct because using a hash prefix of the transaction ID as the first component of a composite primary key distributes writes across multiple tablets in Cloud Spanner, avoiding hot spots on a single tablet. The transaction ID as the second component still enables efficient point lookups by transaction ID, as Spanner can use the hash prefix to locate the tablet and then scan within it for the exact row.
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", "primary". 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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Last reviewed: Jul 4, 2026
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