PCDE Monitor and optimize database performance Practice Question
This PCDE practice question tests your understanding of monitor and optimize database performance. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Exhibit
Refer to the exhibit.
```
Cloud SQL for MySQL instance configuration:
max_connections: 1000
innodb_buffer_pool_size: 26843545600 (25GB)
query_cache_type: 1
query_cache_size: 1048576 (1MB)
```
A DBA notices high query cache lock contention on this Cloud SQL for MySQL instance. Which configuration change should be recommended?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Set query_cache_type=0
Query cache lock contention occurs when multiple threads try to access the query cache simultaneously, causing serialization. Setting `query_cache_type=0` disables the query cache entirely, eliminating the lock contention. This is the recommended fix because MySQL 8.0 deprecated the query cache due to scalability issues, and Cloud SQL for MySQL instances benefit from removing this bottleneck.
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.
Read the scenario before looking for a memorised answer.
✗
Decrease max_connections
Why it's wrong here
Fewer connections does not reduce query cache contention.
✗
Increase innodb_buffer_pool_size
Why it's wrong here
Buffer pool is for data caching, not query cache.
✗
Increase query_cache_size
Why it's wrong here
A larger cache increases lock contention.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that increasing the size of a cache or buffer always improves performance, but here increasing `query_cache_size` exacerbates lock contention, making the problem worse.
Detailed technical explanation
How to think about this question
The MySQL query cache uses a single global mutex to protect all cache operations, meaning only one thread can read or write to it at a time. Under high concurrency, this mutex becomes a severe bottleneck, often causing performance degradation worse than having no cache at all. In MySQL 8.0, the query cache was removed entirely, and Cloud SQL for MySQL instances running MySQL 5.7 should disable it (`query_cache_type=0`) to avoid this contention, relying instead on InnoDB buffer pool and application-level caching.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Monitor and optimize database performance — This question tests Monitor and optimize database performance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Set query_cache_type=0 — Query cache lock contention occurs when multiple threads try to access the query cache simultaneously, causing serialization. Setting `query_cache_type=0` disables the query cache entirely, eliminating the lock contention. This is the recommended fix because MySQL 8.0 deprecated the query cache due to scalability issues, and Cloud SQL for MySQL instances benefit from removing this bottleneck.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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