- A
Use BigQuery to query Bigtable via an external table and run the aggregation there.
Why wrong: This is an external solution, not a schema redesign of the Bigtable table.
- B
Increase the number of Bigtable nodes to improve scan throughput.
Why wrong: Scaling nodes can improve throughput but does not reduce the amount of data scanned; the query still scans all rows.
- C
Add a secondary index on the timestamp column.
Why wrong: Bigtable does not support secondary indexes; row key design is the only access path.
- D
Create a second table with row key 'timestamp#device_id' (with timestamp in natural order) to support time-range queries.
This provides efficient access for the aggregation query by allowing a range scan over the timestamp.
Question 225 of 1,000
Bigtable Multiple Access Patterns: Separate Table Design
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 healthcare analytics company uses Cloud Bigtable to store time-series data from medical devices. The table has a row key of 'device_id#timestamp' where timestamp is stored in reverse order (max - timestamp) so that recent data is at the top. Queries that fetch data for a specific device over a date range are very fast. However, analysts also need to run queries that aggregate data across all devices for a specific hour (e.g., count of readings between 2023-01-01 10:00 and 11:00). These queries are extremely slow because they require scanning all rows. The team must redesign the schema to support both access patterns without duplicating data unnecessarily. What is the best approach?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"best"Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Quick Answer
The answer is to create a second table with a row key of 'timestamp#device_id' using natural timestamp order. This approach directly addresses the need for Bigtable multiple access patterns separate tables, because Bigtable’s sorted row-key structure only supports efficient range scans on the leading key component. The original table optimizes for device-first queries, but the cross-device hourly aggregation requires a time-first key to avoid full-table scans. On the Google Professional Cloud Database Engineer exam, this tests your understanding that Bigtable has no secondary indexes, so you must design separate tables for each distinct access pattern—a common trap is trying to force a single table to serve both queries or assuming you can add an index. A useful memory tip: “One pattern, one table; lead with what you query by.”
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
✓
Create a second table with row key 'timestamp#device_id' (with timestamp in natural order) to support time-range queries.
Option D is correct. Creating a second table with row key 'timestamp#device_id' (with timestamp in natural order) allows efficient range scans for a given time period across all devices, because Bigtable rows are sorted lexicographically by row key. This enables fast aggregation queries without scanning all rows. Option A (using BigQuery external table) would still require scanning the entire Bigtable for each query, and adds latency. Option B (increasing nodes) improves throughput but does not change the need to scan all rows, so it does not address the root cause. Option C (secondary index) is not supported in Bigtable.
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.
- ✗
Use BigQuery to query Bigtable via an external table and run the aggregation there.
Why it's wrong here
This is an external solution, not a schema redesign of the Bigtable table.
- ✗
Increase the number of Bigtable nodes to improve scan throughput.
Why it's wrong here
Scaling nodes can improve throughput but does not reduce the amount of data scanned; the query still scans all rows.
- ✗
Add a secondary index on the timestamp column.
Why it's wrong here
Bigtable does not support secondary indexes; row key design is the only access path.
- ✓
Create a second table with row key 'timestamp#device_id' (with timestamp in natural order) to support time-range queries.
Why this is correct
This provides efficient access for the aggregation query by allowing a range scan over the timestamp.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
- Use explanations to understand the rule behind the answer.
TExam Day Tips
- Underline the problem statement mentally.
- 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 PCDE question test?
Design and implement database schemas — This question tests Design and implement database schemas — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Create a second table with row key 'timestamp#device_id' (with timestamp in natural order) to support time-range queries. — Option D is correct. Creating a second table with row key 'timestamp#device_id' (with timestamp in natural order) allows efficient range scans for a given time period across all devices, because Bigtable rows are sorted lexicographically by row key. This enables fast aggregation queries without scanning all rows. Option A (using BigQuery external table) would still require scanning the entire Bigtable for each query, and adds latency. Option B (increasing nodes) improves throughput but does not change the need to scan all rows, so it does not address the root cause. Option C (secondary index) is not supported in Bigtable.
What should I do if I get this PCDE question wrong?
Identify which PCDE exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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