- A
Use a bit-reversed sequential key generated by the application.
Bit-reversed keys distribute writes evenly while preserving some locality, avoiding hotspots.
- B
Use a UUID string as the primary key.
Why wrong: UUIDs work but are large and inefficient for indexing. A bit-reversed key is preferred.
- C
Continue using sequential IDs from Oracle sequences to maintain consistency.
Why wrong: Sequential IDs cause hotspots in Spanner.
- D
Use a composite key with a hash prefix derived from the transaction timestamp.
Why wrong: While hashing can distribute writes, using a timestamp prefix may not be random enough and could still cause hotspots.
Question 980 of 1,000
PCDOE Design and Plan Database Solutions Practice Question
This PCDOE practice question tests your understanding of design and plan database solutions. Compare every option against the stated constraints before choosing — the best answer satisfies all requirements, not just the most obvious one. 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 is migrating from an on-premises Oracle RAC database to Cloud Spanner. The current application uses sequences to generate globally unique IDs for transactions. To avoid creating hotspots in Spanner, the database architect recommends using a different primary key strategy. Which primary key design is most appropriate for Spanner to avoid hotspots?
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 bit-reversed sequential key generated by the application.
Option A is correct because bit-reversed sequential keys distribute writes evenly across Cloud Spanner's split boundaries, preventing hotspots. Spanner uses key-range-based sharding, so monotonically increasing keys (like Oracle sequences) cause all new writes to hit a single split, leading to contention. Bit-reversal spreads sequential values across the key space, ensuring balanced write distribution.
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 bit-reversed sequential key generated by the application.
Why this is correct
Bit-reversed keys distribute writes evenly while preserving some locality, 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 a UUID string as the primary key.
Why it's wrong here
UUIDs work but are large and inefficient for indexing. A bit-reversed key is preferred.
- ✗
Continue using sequential IDs from Oracle sequences to maintain consistency.
Why it's wrong here
Sequential IDs cause hotspots in Spanner.
- ✗
Use a composite key with a hash prefix derived from the transaction timestamp.
Why it's wrong here
While hashing can distribute writes, using a timestamp prefix may not be random enough and could still cause hotspots.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that UUIDs are always the best choice for distributed databases, but in Spanner, they cause storage bloat and poor performance; the trap here is that candidates may overlook the specific hotspot issue with sequential keys and choose UUIDs for their uniqueness without considering Spanner's key-range sharding behavior.
Detailed technical explanation
How to think about this question
Spanner uses a distributed, Paxos-based storage system where data is partitioned into splits based on key ranges. Bit-reversed sequential keys transform a monotonically increasing integer (e.g., 1, 2, 3) into values that are spread across the key space (e.g., 1 → 2^63, 2 → 2^62), ensuring that consecutive writes go to different splits. This technique is analogous to using a 'reverse' function on the key to avoid the 'hotspot' problem common in NoSQL and distributed databases like Bigtable.
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.
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FAQ
Questions learners often ask
What does this PCDOE question test?
Design and Plan Database Solutions — This question tests Design and Plan Database Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a bit-reversed sequential key generated by the application. — Option A is correct because bit-reversed sequential keys distribute writes evenly across Cloud Spanner's split boundaries, preventing hotspots. Spanner uses key-range-based sharding, so monotonically increasing keys (like Oracle sequences) cause all new writes to hit a single split, leading to contention. Bit-reversal spreads sequential values across the key space, ensuring balanced write distribution.
What should I do if I get this PCDOE 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
This PCDOE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCDOE exam.
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