Google PCA Practice Question: Analysing and Optimising Technical and Business Processes
An engineer is designing a Bigtable schema for time-series data consisting of sensor readings. Each sensor emits a reading every second. The access pattern is to retrieve all readings for a specific sensor within a time range. Which row key design will provide the best performance?
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
✓
Use row key: [sensor_id]#[reverse_timestamp]
Bigtable stores rows sorted by key. A row key structured as [sensor_id]#[reverse_timestamp] ensures that all data for a sensor is contiguous, and sorting by reverse timestamp allows recent data to be retrieved first. A single row key per sensor with column qualifiers would cause hotspots and limit scalability.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use row key: [sensor_id]#[reverse_timestamp]
Why this is correct
This ensures all readings for a sensor are close together and sorted by timestamp, optimizing range scans.
- ✗
Use a single row per sensor with column qualifiers as timestamps
Why it's wrong here
This would create a wide row that can exceed limits and cause hotspots on the sensor node.
- ✗
Use timestamp as the row key and sensor ID as column qualifier
Why it's wrong here
This would spread each sensor's data across many tablets, hurting scan performance.
- ✗
Use a random prefix to distribute writes evenly
Why it's wrong here
Random prefix distributes writes but scatters sensor data across tablets, making range reads inefficient.
Go deeper
Related to this question
Learn chapter
Identity and Access Management (IAM)
Key term
Row
A row is a horizontal record in a database table that contains all the information about a single entity, like one customer or one product.
Key term
Data
Data is raw, unprocessed information, like numbers, words, or measurements, that can be stored, processed, and analyzed by computers.
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