PCDE Design and implement database schemas Practice Question

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?

⚠ Common exam trap

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.

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

✓

sensor_id#reverse_timestamp

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.

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#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.
✗
random_UUID
Why it's wrong here
No locality for sensor or time queries.

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PCDE Design and implement database schemas Practice Question

Why each option matters

Option-by-option breakdown

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