PCDE Monitor and optimize database performance Practice Question
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 Bigtable instance stores time-series data with row keys formatted as `#deviceID#timestamp`. The application often queries recent data for a specific device. Monitoring shows high read latency when scanning multiple devices. The row key design is causing hotspotting. What is the best redesign?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a hash prefix before deviceID.
Option B is correct because prepending a hash prefix (e.g., a cryptographic hash of the deviceID) to the row key distributes writes and reads evenly across Bigtable tablets, eliminating hotspotting caused by sequential deviceID-based keys. This ensures that queries for recent data (which would otherwise concentrate on a single tablet) are spread across multiple nodes, reducing read latency.
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 separate tables per device.
Why it's wrong here
Managing many tables is inefficient and does not scale well; it also does not address query performance across devices.
✓
Use a hash prefix before deviceID.
Why this is correct
Hashing distributes rows evenly across the keyspace, alleviating hotspotting while preserving locality for device queries.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Store timestamps in column qualifiers.
Why it's wrong here
This would not affect row key distribution and could lead to complex schema and performance issues.
✗
Prefix the row key with the deviceID and reverse the timestamp.
Why it's wrong here
This still prefixes with deviceID, causing hotspotting if a device is accessed frequently.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that reversing the timestamp (option D) solves hotspotting, but candidates fail to realize that the leading key component (deviceID) is still sequential, so all recent data for a device remains on the same tablet, and only a hash prefix (option B) truly distributes the load.
Detailed technical explanation
How to think about this question
Under the hood, Bigtable splits tablets based on row key ranges; sequential keys (e.g., deviceID#timestamp) cause all writes for a device to land on the same tablet, leading to a 'hot spot'. A hash prefix (e.g., MD5 or SHA-1 of deviceID) ensures uniform distribution across tablets, but queries must then scan all tablets for a specific device; this is acceptable because the read latency improvement from eliminating hotspotting outweighs the cost of a scatter-gather query. In practice, a common pattern is to use a hash prefix of the deviceID and then append the timestamp, allowing efficient range scans for a device by hashing the deviceID and scanning that specific hash prefix range.
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.
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: Use a hash prefix before deviceID. — Option B is correct because prepending a hash prefix (e.g., a cryptographic hash of the deviceID) to the row key distributes writes and reads evenly across Bigtable tablets, eliminating hotspotting caused by sequential deviceID-based keys. This ensures that queries for recent data (which would otherwise concentrate on a single tablet) are spread across multiple nodes, reducing read latency.
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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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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