- A
Partition the table by customer_id to reduce the number of secondary indexes needed.
Why wrong: Partitioning does not directly reduce the number of secondary indexes; it may even require additional indexes on partition keys.
- B
Create a secondary index on the composite key to keep the same query performance.
Why wrong: Adding a secondary index on the same composite key would increase storage and maintenance overhead without reducing costs.
- C
Replace the composite primary key with a surrogate UUID primary key and add unique constraints on the original columns.
A UUID primary key is smaller than a composite key, and unique constraints enforce data integrity without the overhead of a clustered index.
- D
Use the CLUSTER command to physically reorder the table based on the composite key.
Why wrong: Cloud SQL for PostgreSQL does not support the CLUSTER command; it is not available in managed environments.
Quick Answer
The correct choice is to replace the composite primary key with a surrogate UUID primary key and add unique constraints on the original columns. This works because in PostgreSQL, every secondary index automatically includes the primary key columns as row identifiers, so a wide composite key on (customer_id, order_date) bloats all secondary indexes, increasing storage and I/O costs. A narrower UUID surrogate key shrinks those secondary indexes, directly reducing cost—a critical consideration for the Google Professional Cloud Database Engineer exam, which frequently tests how schema design impacts Cloud SQL performance and pricing. A common trap is assuming composite keys are always optimal for query performance, but the exam emphasizes that secondary index size matters more for cost efficiency in managed services. Memory tip: “UUID shrinks the index bloat” – think of the surrogate key as a slim pointer that keeps every secondary index lean.
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 team is migrating an on-premises PostgreSQL database to Cloud SQL. The current schema uses a composite primary key on columns (customer_id, order_date) in the orders table. The migration team wants to reduce the cost of secondary indexes. Which schema design change should they consider?
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
Replace the composite primary key with a surrogate UUID primary key and add unique constraints on the original columns.
Option C is correct because replacing the composite primary key with a surrogate UUID primary key reduces the size of secondary indexes. In PostgreSQL (and Cloud SQL), secondary indexes include the primary key columns as row identifiers. A composite key on (customer_id, order_date) is wide, making every secondary index large and costly. A UUID surrogate key is narrower, shrinking all secondary indexes and reducing storage and I/O costs.
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.
- ✗
Partition the table by customer_id to reduce the number of secondary indexes needed.
Why it's wrong here
Partitioning does not directly reduce the number of secondary indexes; it may even require additional indexes on partition keys.
- ✗
Create a secondary index on the composite key to keep the same query performance.
Why it's wrong here
Adding a secondary index on the same composite key would increase storage and maintenance overhead without reducing costs.
- ✓
Replace the composite primary key with a surrogate UUID primary key and add unique constraints on the original columns.
Why this is correct
A UUID primary key is smaller than a composite key, and unique constraints enforce data integrity without the overhead of a clustered index.
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 the CLUSTER command to physically reorder the table based on the composite key.
Why it's wrong here
Cloud SQL for PostgreSQL does not support the CLUSTER command; it is not available in managed environments.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that partitioning or clustering reduces index storage costs, when in fact only narrowing the primary key (or using a surrogate key) directly shrinks secondary index size in PostgreSQL.
Trap categories for this question
Command / output trap
Cloud SQL for PostgreSQL does not support the CLUSTER command; it is not available in managed environments.
Detailed technical explanation
How to think about this question
In PostgreSQL, secondary indexes (B-tree) store a copy of the indexed columns plus a reference to the heap tuple, which includes the primary key columns. A wide composite primary key bloats every secondary index entry. Using a UUID (16 bytes) as a surrogate key minimizes this overhead. Additionally, UUIDs can be generated client-side to avoid contention, but they may cause index fragmentation; using sequential UUIDs (e.g., UUID v7) can mitigate this in high-write workloads.
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: Replace the composite primary key with a surrogate UUID primary key and add unique constraints on the original columns. — Option C is correct because replacing the composite primary key with a surrogate UUID primary key reduces the size of secondary indexes. In PostgreSQL (and Cloud SQL), secondary indexes include the primary key columns as row identifiers. A composite key on (customer_id, order_date) is wide, making every secondary index large and costly. A UUID surrogate key is narrower, shrinking all secondary indexes and reducing storage and I/O costs.
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: Jun 30, 2026
This PCDE 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 PCDE exam.
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