- A
Sensor ID (raw) as the only key
Why wrong: If sensor ID is the only key, all writes per sensor go to one node.
- B
Sensor location as a column
Why wrong: Location is a column, not row key component.
- C
Reversed timestamp
Reversed timestamp spreads writes and allows recent-first queries.
- D
Timestamp in natural order
Why wrong: Natural order timestamps cause write hotspots on current time.
- E
Hash of sensor ID as prefix
Salting distributes writes across nodes.
Question 526 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.
An engineer is designing a Bigtable schema for a weather data application. The data is written by thousands of sensors, each generating a reading every minute. Queries typically retrieve all readings for a sensor in a time range. The row key should be designed to avoid hotspots and support these queries. Which two row key components are recommended? (Choose two.)
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
✓
Reversed timestamp
Option C is correct because using a reversed timestamp (e.g., Long.MAX_VALUE - timestamp) as part of the row key spreads writes across Bigtable tablets, avoiding hotspots that occur when sensors write sequentially in natural time order. This design also supports efficient range scans for a sensor's data over a time range when combined with a sensor ID prefix.
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 (raw) as the only key
Why it's wrong here
If sensor ID is the only key, all writes per sensor go to one node.
- ✗
Sensor location as a column
Why it's wrong here
Location is a column, not row key component.
- ✓
Reversed timestamp
Why this is correct
Reversed timestamp spreads writes and allows recent-first queries.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Timestamp in natural order
Why it's wrong here
Natural order timestamps cause write hotspots on current time.
- ✓
Hash of sensor ID as prefix
Why this is correct
Salting distributes writes across nodes.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that natural-order timestamps are optimal for time-range queries, but the trap here is that they cause write hotspots in Bigtable, so a reversed timestamp is required to distribute writes while still supporting range scans.
Detailed technical explanation
How to think about this question
Bigtable automatically splits tablets at row key boundaries, so a hash prefix (e.g., MD5 or CRC32 of sensor ID) distributes writes uniformly across tablets, while the reversed timestamp enables efficient range scans for recent data by keeping them contiguous. In practice, a row key like 'hash(sensor_id)#reversed_timestamp' balances write distribution and query performance, and the hash prefix length can be tuned to match the number of tablets.
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.
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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: Reversed timestamp — Option C is correct because using a reversed timestamp (e.g., Long.MAX_VALUE - timestamp) as part of the row key spreads writes across Bigtable tablets, avoiding hotspots that occur when sensors write sequentially in natural time order. This design also supports efficient range scans for a sensor's data over a time range when combined with a sensor ID prefix.
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.
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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