The correct answer is to use a hash of the original key as a prefix to the primary key. This design avoids hotspotting in Cloud Spanner by ensuring that write operations are distributed evenly across the database’s splits, rather than concentrating on a single node. When using sequential keys from Oracle sequences, Cloud Spanner’s append-only storage model forces all new rows into the same split, creating a performance bottleneck known as a hotspot. The hash prefix spreads the write load while still allowing related rows to remain co-located for efficient queries. On the Google Professional Cloud Database Engineer exam, this question tests your understanding of distributed database design and the specific anti-pattern of sequential keys in Cloud Spanner. A common trap is assuming that any unique key works, but the key insight is that monotonic increases cause hotspotting. Memory tip: think “hash to scatter, prefix to gather” — the hash scatters writes, and the prefix keeps related data together.
PCDE Plan and manage database infrastructure Practice Question
This PCDE practice question tests your understanding of plan and manage database infrastructure. 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 is migrating a large Oracle database to Cloud Spanner. The source database uses sequences for primary key generation. The database engineer needs to design the Cloud Spanner schema to avoid hotspotting. What primary key design should they recommend?
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.
Why wrong: Sequence-based keys are monotonically increasing and will cause hotspotting in Spanner.
B
Use a composite primary key with a timestamp prefix.
Why wrong: Timestamp prefixes are monotonically increasing and will cause hotspotting.
C
Use a hash of the original key as a prefix to the primary key.
A hash prefix distributes the write load evenly across splits, avoiding hotspots.
D
Use UUIDs as the primary key without modification.
Why wrong: UUIDs are random and avoid hotspotting, but they are not native to Spanner and would be stored as strings, which could cause performance issues.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a hash of the original key as a prefix to the primary key.
Option C is correct because using a hash of the original key as a prefix to the primary key distributes writes evenly across Cloud Spanner's splits, preventing hotspotting. Cloud Spanner uses a distributed, append-only storage model where sequential keys (like from Oracle sequences) cause all new writes to land on the same split, creating a hotspot. A hash prefix ensures that related rows are still co-located for efficient queries while spreading write load across multiple nodes.
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.
✗
Keep the same sequence-based integer keys.
Why it's wrong here
Sequence-based keys are monotonically increasing and will cause hotspotting in Spanner.
✗
Use a composite primary key with a timestamp prefix.
Why it's wrong here
Timestamp prefixes are monotonically increasing and will cause hotspotting.
✓
Use a hash of the original key as a prefix to the primary key.
Why this is correct
A hash prefix distributes the write load evenly across 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.
✗
Use UUIDs as the primary key without modification.
Why it's wrong here
UUIDs are random and avoid hotspotting, but they are not native to Spanner and would be stored as strings, which could cause performance issues.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that UUIDs or timestamps inherently solve hotspotting, but the trap here is that only a hash prefix (or similar distribution mechanism) guarantees even write distribution across Cloud Spanner's splits.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed storage architecture where data is partitioned into splits based on the primary key range. When using a hash prefix (e.g., SHA256 of the original key modulo a large number), the leading bytes of the key become uniformly distributed, ensuring that writes are spread across all splits. This technique is recommended in Google Cloud documentation for avoiding hotspotting, especially when migrating from databases with sequential key generation like Oracle sequences.
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.
Plan and manage database infrastructure — This question tests Plan and manage database infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a hash of the original key as a prefix to the primary key. — Option C is correct because using a hash of the original key as a prefix to the primary key distributes writes evenly across Cloud Spanner's splits, preventing hotspotting. Cloud Spanner uses a distributed, append-only storage model where sequential keys (like from Oracle sequences) cause all new writes to land on the same split, creating a hotspot. A hash prefix ensures that related rows are still co-located for efficient queries while spreading write load across multiple nodes.
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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