Quick Answer
The correct answer is user_id#timestamp and user_id#activity_type#timestamp. Both designs leverage Bigtable’s lexicographic ordering by placing the user ID first, which groups all rows for a given user together and allows efficient row range scans when combined with a timestamp filter for time-based queries. The key technical concept is that Bigtable row keys are sorted and scanned as byte strings, so a user ID prefix ensures all activities for that user are contiguous, while the timestamp suffix provides natural ordering and uniqueness. On the Google Professional Cloud Database Engineer exam, this tests your understanding of how to model time-series data for point-in-time retrieval, with a common trap being designs that place timestamp first—those scatter data across regions and kill scan performance. Remember the memory tip: “User first, time last—scan fast, don’t blast.”
This PCDE practice question tests your understanding of design and implement database schemas. 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.
A team is designing a schema for a user activity logging system using Bigtable. Each log entry includes a user ID, activity type, timestamp, and details. The access pattern requires retrieving all activities for a specific user within a time range. Which TWO row key designs are suitable? (Choose TWO.)
timestamp#user_id
Why wrong: Scatters activities across the table; inefficient for per-user queries.
random_uuid
Why wrong: No locality; cannot do range scans.
reverse_timestamp
Why wrong: Does not group by user; requires scanning entire table.
user_id#activity_type#timestamp
Allows filtering by activity type within a user.
user_id#timestamp
Groups all activities for a user together in time order.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
user_id#activity_type#timestamp
Option D (user_id#activity_type#timestamp) is correct because it groups all activities for a user under a single row key prefix, enabling efficient row range scans for a specific user. The activity_type suffix allows filtering by activity type if needed, while the timestamp ensures uniqueness and ordered storage. Option E (user_id#timestamp) is correct because it directly supports the access pattern of retrieving all activities for a user within a time range by scanning rows with the user_id prefix and filtering on the timestamp component.
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#user_id
Why it's wrong here
Scatters activities across the table; inefficient for per-user queries.
random_uuid
Why it's wrong here
No locality; cannot do range scans.
reverse_timestamp
Why it's wrong here
Does not group by user; requires scanning entire table.
user_id#activity_type#timestamp
Why this is correct
Allows filtering by activity type within a user.
Related concept
Read the scenario before looking for a memorised answer.
user_id#timestamp
Why this is correct
Groups all activities for a user together in time order.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Google Cloud often tests the misconception that a timestamp-first key is optimal for time-range queries, but the actual requirement is user-specific retrieval, which demands a user-first key design to avoid full-table scans.
Detailed technical explanation
In Bigtable, row keys are sorted lexicographically, so a prefix like user_id#timestamp ensures all rows for a user are contiguous, allowing efficient range scans with start and end keys. The timestamp component can be stored as a string in ISO 8601 format or as a reversed integer to enable descending order scans. Real-world implementations often use a hashed prefix to avoid hot-spotting on high-traffic users, but the core design principle remains prefix-based grouping for access patterns.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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: user_id#activity_type#timestamp — Option D (user_id#activity_type#timestamp) is correct because it groups all activities for a user under a single row key prefix, enabling efficient row range scans for a specific user. The activity_type suffix allows filtering by activity type if needed, while the timestamp ensures uniqueness and ordered storage. Option E (user_id#timestamp) is correct because it directly supports the access pattern of retrieving all activities for a user within a time range by scanning rows with the user_id prefix and filtering on the timestamp component.
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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