- A
Remove the foreign key constraints and enforce referential integrity in the application logic instead.
Foreign key constraints require a lookup on the parent table for every INSERT, causing latency. Removing them reduces write overhead, though integrity must be ensured by the application.
- B
Remove the secondary index on account_id because it adds write overhead.
Why wrong: Removing the secondary index would speed up INSERTs but degrade SELECT performance for account lookups, which is likely needed.
- C
Change the primary key to (account_id, transaction_id) to avoid secondary index overhead.
Why wrong: Reordering the primary key does not eliminate foreign key checking overhead; it only changes the clustering order.
- D
Convert the table to a temporal table with system-versioning to avoid constraint checking.
Why wrong: Cloud SQL for MySQL does not support temporal tables; this feature is not available.
Quick Answer
The answer is to remove the foreign key constraints and enforce referential integrity in the application logic instead. This is correct because every INSERT into a table with foreign key constraints triggers an internal lookup on the parent table to verify the referenced key exists, creating a latency penalty that scales with the size of the parent table—a critical factor when migrating from Compute Engine to Cloud SQL for MySQL. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of how foreign key constraints impact insert performance in Cloud SQL, often appearing as a trap where candidates mistakenly add indexes or adjust buffer pools instead of recognizing the constraint overhead. The key insight is that Cloud SQL’s managed environment does not eliminate the fundamental MySQL cost of referential checks. Memory tip: “Constraints check, inserts wreck”—if you need raw insert speed, move integrity checks to the app layer.
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 financial services company runs a MySQL database on Compute Engine. They want to migrate to Cloud SQL for MySQL to reduce operational overhead. The current schema includes a table 'transactions' with a composite primary key on (transaction_id, account_id) and a secondary index on account_id for account lookups. The database also uses foreign key constraints to ensure referential integrity between 'transactions' and 'accounts'. During migration testing, they observe that INSERT operations on 'transactions' are slower than expected. What schema change should they implement to improve INSERT performance in Cloud SQL?
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
Remove the foreign key constraints and enforce referential integrity in the application logic instead.
Foreign key constraints in MySQL (including Cloud SQL) require an internal check on every INSERT to verify that the referenced parent key exists. This adds a latency penalty proportional to the size of the parent table. Removing the constraint and moving referential integrity to the application eliminates this per-row check, directly improving INSERT throughput.
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.
- ✓
Remove the foreign key constraints and enforce referential integrity in the application logic instead.
Why this is correct
Foreign key constraints require a lookup on the parent table for every INSERT, causing latency. Removing them reduces write overhead, though integrity must be ensured by the application.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Remove the secondary index on account_id because it adds write overhead.
Why it's wrong here
Removing the secondary index would speed up INSERTs but degrade SELECT performance for account lookups, which is likely needed.
- ✗
Change the primary key to (account_id, transaction_id) to avoid secondary index overhead.
Why it's wrong here
Reordering the primary key does not eliminate foreign key checking overhead; it only changes the clustering order.
- ✗
Convert the table to a temporal table with system-versioning to avoid constraint checking.
Why it's wrong here
Cloud SQL for MySQL does not support temporal tables; this feature is not available.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that secondary indexes are the primary cause of write slowdowns, when in reality foreign key constraint checks are far more expensive per row than index maintenance.
Detailed technical explanation
How to think about this question
Under the hood, MySQL enforces foreign keys using a row-level shared lock on the parent table during INSERT, which can cause contention under concurrent load. In Cloud SQL, the InnoDB storage engine also performs an index lookup on the parent table's primary key for each foreign key check. In high-volume transactional systems, this overhead can become a bottleneck, making application-level enforcement a common optimization pattern.
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
An e-commerce site experiences heavy traffic on Black Friday and near-zero traffic during off-peak weeks. Rather than provisioning permanent large VMs, the team uses auto-scaling groups that add capacity automatically under load and reduce it overnight. Questions like this test whether you understand elasticity, availability zones, and cloud compute scaling patterns.
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: Remove the foreign key constraints and enforce referential integrity in the application logic instead. — Foreign key constraints in MySQL (including Cloud SQL) require an internal check on every INSERT to verify that the referenced parent key exists. This adds a latency penalty proportional to the size of the parent table. Removing the constraint and moving referential integrity to the application eliminates this per-row check, directly improving INSERT throughput.
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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