- A
[sensor_id]#[timestamp]
Why wrong: Without salting, writes for the same sensor may hotspot on a single tablet server if sensor IDs are sequential.
- B
[salted_hash]#[sensor_id]#[reversed_timestamp]
Salting distributes writes, sensor ID groups data, reversed timestamp enables efficient time-range queries.
- C
[timestamp]#[sensor_id]
Why wrong: This leads to all writes hitting the same region for the current timestamp, causing hotspots.
- D
[sensor_id]#[reversed_timestamp]
Why wrong: Lacks salting; if sensor IDs are sequential, writes may hotspot.
Question 280 of 1,000
PCDOE Design and Plan Database Solutions Practice Question
This PCDOE practice question tests your understanding of design and plan database solutions. 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.
You are designing a Cloud Bigtable schema for a time-series application that stores temperature readings from sensors. Each reading has a sensor ID (string), a timestamp (microseconds), and a temperature value. Queries always filter by sensor ID and a time range. Which row key design is optimal?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"always"Why it matters: Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
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
✓
[salted_hash]#[sensor_id]#[reversed_timestamp]
Option B is optimal because it uses a salted hash to distribute writes across Bigtable tablets, avoiding hot-spotting on a single node for high-write sensors. The sensor_id ensures all data for a sensor is co-located for efficient range scans, and the reversed timestamp allows queries for the most recent data to be served from the start of the row range, leveraging Bigtable's lexicographic ordering.
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.
- ✗
[sensor_id]#[timestamp]
Why it's wrong here
Without salting, writes for the same sensor may hotspot on a single tablet server if sensor IDs are sequential.
- ✓
[salted_hash]#[sensor_id]#[reversed_timestamp]
Why this is correct
Salting distributes writes, sensor ID groups data, reversed timestamp enables efficient time-range queries.
Clue confirmation
The clue word "always" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
[timestamp]#[sensor_id]
Why it's wrong here
This leads to all writes hitting the same region for the current timestamp, causing hotspots.
- ✗
[sensor_id]#[reversed_timestamp]
Why it's wrong here
Lacks salting; if sensor IDs are sequential, writes may hotspot.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a simple sensor_id prefix is sufficient for time-series data, ignoring the critical need for write distribution via salting to avoid hot-spotting in high-throughput scenarios.
Detailed technical explanation
How to think about this question
Bigtable stores rows in lexicographic order by row key, and a single tablet server handles a contiguous range of keys. Without salting, all writes for a popular sensor_id land on one tablet, causing CPU and I/O bottlenecks. The salted hash (e.g., a 2-byte hash prefix) spreads writes across tablets, while the sensor_id and reversed_timestamp keep related rows together for efficient time-range scans. In practice, the salt can be computed as crc32(sensor_id) % 1000 to ensure even distribution.
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.
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FAQ
Questions learners often ask
What does this PCDOE question test?
Design and Plan Database Solutions — This question tests Design and Plan Database Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: [salted_hash]#[sensor_id]#[reversed_timestamp] — Option B is optimal because it uses a salted hash to distribute writes across Bigtable tablets, avoiding hot-spotting on a single node for high-write sensors. The sensor_id ensures all data for a sensor is co-located for efficient range scans, and the reversed timestamp allows queries for the most recent data to be served from the start of the row range, leveraging Bigtable's lexicographic ordering.
What should I do if I get this PCDOE 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: "always". Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jul 4, 2026
This PCDOE 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 PCDOE exam.
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