This PCDE practice question tests your understanding of design and implement database schemas. Compare every option against the stated constraints before choosing — the best answer satisfies all requirements, not just the most obvious one. 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 team is designing a schema for a time-series database in Bigtable to store IoT sensor readings. Each sensor sends a reading every minute. The team needs to create a row key that supports efficient queries for a specific sensor's readings over a time range. Which row key design is most appropriate?
timestamp#sensor_id
Why wrong: Reads for a single sensor would be scattered across the table.
hash(sensor_id)#timestamp
Why wrong: Hash destroys ordering, making time range scans inefficient.
sensor_id#reverse_timestamp
Groups all readings for a sensor together in reverse chronological order.
random_UUID
Why wrong: No locality for sensor or time queries.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
sensor_id#reverse_timestamp
Option C is correct because Bigtable stores rows sorted lexicographically by row key. By placing the sensor_id first, all readings for a given sensor are co-located in contiguous rows. Using reverse_timestamp (e.g., 9999-12-31 minus actual timestamp) ensures that the most recent readings appear first within that sensor's row range, which optimizes scans for the latest data and allows efficient range queries over a time window.
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.
timestamp#sensor_id
Why it's wrong here
Reads for a single sensor would be scattered across the table.
hash(sensor_id)#timestamp
Why it's wrong here
Hash destroys ordering, making time range scans inefficient.
sensor_id#reverse_timestamp
Why this is correct
Groups all readings for a sensor together in reverse chronological order.
Related concept
Read the scenario before looking for a memorised answer.
random_UUID
Why it's wrong here
No locality for sensor or time queries.
Common exam traps
Google Cloud often tests the misconception that putting the timestamp first is always best for time-range queries, but in Bigtable, the row key's prefix determines data locality, so the sensor_id must come first to avoid scattering reads across the entire table.
Detailed technical explanation
Bigtable's row key design leverages lexicographic sorting to enable prefix scans. By using reverse_timestamp (e.g., Long.MAX_VALUE - epoch_millis), the most recent data is at the top of the scan, which is critical for real-time dashboards that need the latest sensor readings first. This pattern also avoids hot-spotting on a single tablet server when many sensors write simultaneously, as long as the sensor_id prefix provides sufficient distribution.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
What to study next
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FAQ
Design and implement database schemas — This question tests Design and implement database schemas — Read the scenario before looking for a memorised answer..
The correct answer is: sensor_id#reverse_timestamp — Option C is correct because Bigtable stores rows sorted lexicographically by row key. By placing the sensor_id first, all readings for a given sensor are co-located in contiguous rows. Using reverse_timestamp (e.g., 9999-12-31 minus actual timestamp) ensures that the most recent readings appear first within that sensor's row range, which optimizes scans for the latest data and allows efficient range queries over a time window.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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