Quick Answer
The answer is a long-running query executing on the replica. Even with a constant write workload on the primary, a slow query on the replica blocks the single-threaded SQL thread from applying relay log events, causing replica lag to increase. In MySQL’s default replication, the SQL thread processes events sequentially; if one query takes minutes, all subsequent events queue up behind it, creating a growing lag that is independent of the primary’s write rate. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding that replica lag is not always a primary-side issue—it often stems from replica-side contention. A common trap is assuming the primary’s workload must be spiking, but the key insight is that a constant workload reveals a bottleneck on the replica itself. Memory tip: “Single thread, single dread”—one slow query on the replica stalls the entire replication pipeline.
This PCDE practice question tests your understanding of monitor and optimize database performance. 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 Cloud SQL for MySQL instance is experiencing increased replica lag. The write workload is constant. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Binary logs are being deleted too frequently on the primary
Why wrong: Deleting binary logs would break replication, not cause lag.
The query cache is enabled on the primary
Why wrong: Query cache does not affect replication lag.
The replica is on a higher machine tier than the primary
Why wrong: A higher tier replica should lag less.
A long-running query is executing on the replica
Long queries delay the SQL thread from applying changes.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
A long-running query is executing on the replica
A long-running query on the replica can block the SQL thread from applying relay log events, causing replica lag to increase even if the primary's write workload is constant. This is because replication is single-threaded by default in MySQL, so one slow query stalls all subsequent events until it completes.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Binary logs are being deleted too frequently on the primary
Why it's wrong here
Deleting binary logs would break replication, not cause lag.
The query cache is enabled on the primary
Why it's wrong here
Query cache does not affect replication lag.
The replica is on a higher machine tier than the primary
Why it's wrong here
A higher tier replica should lag less.
A long-running query is executing on the replica
Why this is correct
Long queries delay the SQL thread from applying changes.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Google Cloud often tests the misconception that replica lag is always caused by primary-side issues (like binary log deletion or query cache), when in fact the replica's own processing bottlenecks are a frequent root cause.
Detailed technical explanation
In MySQL replication, the replica uses two threads: the I/O thread fetches binary log events from the primary and writes them to the relay log, while the SQL thread reads and applies them. A long-running query on the replica blocks the SQL thread, causing the relay log to accumulate and lag to grow. This is a common issue in single-threaded replication; using multi-threaded replication (slave_parallel_workers) can mitigate it by allowing parallel apply of events from different databases.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
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FAQ
Monitor and optimize database performance — This question tests Monitor and optimize database performance — Read the scenario before looking for a memorised answer..
The correct answer is: A long-running query is executing on the replica — A long-running query on the replica can block the SQL thread from applying relay log events, causing replica lag to increase even if the primary's write workload is constant. This is because replication is single-threaded by default in MySQL, so one slow query stalls all subsequent events until it completes.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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