- A
timestamp#sensorID
Why wrong: Does not group by sensor.
- B
sensorID#timestamp
Why wrong: Most recent data is scattered if timestamp is ascending.
- C
sensorID#reverse_timestamp
Groups by sensor and puts recent data first.
- D
hash(sensorID)#timestamp
Why wrong: Distributes writes well but makes range scans inefficient.
Question 952 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 need to design a Bigtable row key for a time-series application that records temperature readings from thousands of sensors. The most common query is 'get all readings for a specific sensor in the last hour'. Which row key design is optimal?
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
✓
sensorID#reverse_timestamp
Option C is optimal because it groups all readings for a sensor together (via sensorID as the row key prefix) while using reverse timestamps to ensure the most recent data appears first within each row. This design allows Bigtable to efficiently scan a single row for the last hour's readings using a prefix scan on sensorID with a timestamp range filter, minimizing the number of rows accessed.
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.
- ✗
timestamp#sensorID
Why it's wrong here
Does not group by sensor.
- ✗
sensorID#timestamp
Why it's wrong here
Most recent data is scattered if timestamp is ascending.
- ✓
sensorID#reverse_timestamp
Why this is correct
Groups by sensor and puts recent data first.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
hash(sensorID)#timestamp
Why it's wrong here
Distributes writes well but makes range scans inefficient.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose sensorID#timestamp (Option B) thinking it groups data correctly, but they overlook that Bigtable's lexicographic ordering places older data first, making 'last hour' queries require scanning the entire row or using a reverse scan, which is less efficient than reverse timestamps.
Detailed technical explanation
How to think about this question
Bigtable stores rows in lexicographic order by row key, so sensorID#reverse_timestamp ensures that all data for a sensor is contiguous and the newest entries are at the top of the row. Reverse timestamps are typically implemented by subtracting the Unix timestamp from a large constant (e.g., Long.MAX_VALUE - timestamp) to invert the sort order. In real-world IoT deployments, this design reduces read latency from seconds to milliseconds for time-range queries on high-frequency sensor data.
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
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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: sensorID#reverse_timestamp — Option C is optimal because it groups all readings for a sensor together (via sensorID as the row key prefix) while using reverse timestamps to ensure the most recent data appears first within each row. This design allows Bigtable to efficiently scan a single row for the last hour's readings using a prefix scan on sensorID with a timestamp range filter, minimizing the number of rows accessed.
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.
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Last reviewed: Jul 4, 2026
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