The answer is to add a secondary index on Timestamp. This is correct because Cloud Spanner range scan performance is directly tied to primary key order, as data is physically sorted by the leading key columns. When Timestamp is not the leading column, a range query filtering only by Timestamp across all users forces a full table scan, but a secondary index on Timestamp creates a separate sorted structure optimized for that access pattern. On the Google Professional Cloud Database Engineer exam, this tests your understanding of how Spanner’s distributed storage architecture leverages key order for efficient scans, and a common trap is assuming any index on Timestamp works—only a leading-key index or a secondary index on the filter column provides contiguous reads. A useful memory tip: “Lead with the filter to avoid the scatter.”
PCDE Monitor and optimize database performance Practice Question
This PCDE practice question tests your understanding of monitor and optimize database performance. 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 Cloud Spanner database has a table with a primary key (UserId, Timestamp). Queries that filter by Timestamp range for a specific UserId are fast, but queries that filter only by Timestamp range across all users are slow. Which TWO improvements would help?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "primary"
Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a leading column of the primary key that supports range scans.
Option A is correct because in Cloud Spanner, the primary key order determines how data is physically sorted and stored. By making Timestamp the leading column of the primary key (e.g., (Timestamp, UserId)), range scans on Timestamp become efficient as Spanner can perform a contiguous scan of the sorted data. This directly addresses the slow queries that filter only by Timestamp range across all users.
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 a leading column of the primary key that supports range scans.
Why this is correct
Redesigning the primary key with Timestamp as the first column allows efficient range scans across users.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use a hash prefix on UserId.
Why it's wrong here
Hashing UserId does not help range scans on Timestamp; it may even worsen performance.
✗
Create an interleaved table structure.
Why it's wrong here
Interleaving is for parent-child relationships and does not improve range scans across users.
✓
Add a secondary index on Timestamp.
Why this is correct
A secondary index on Timestamp allows direct range scans without scanning the entire table.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Partition the table by Timestamp.
Why it's wrong here
Cloud Spanner does not support partitioning by a column; splits are based on primary key order.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that adding a secondary index is always the best solution, but here both a leading key column change and a secondary index are valid; the trap is that candidates might think partitioning (Option E) is supported in Spanner when it is not.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed, globally replicated storage system where data is split into tablets based on the primary key order. When Timestamp is not the leading column, a full table scan is required for timestamp-only queries because the data is sorted first by UserId. Adding a secondary index on Timestamp (Option D) creates a separate index table sorted by Timestamp, allowing efficient range scans without altering the base table's primary key. This is a common pattern for optimizing non-leading key range queries in Spanner.
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.
TExam Day Tips
→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 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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Monitor and optimize database performance — This question tests Monitor and optimize database performance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use a leading column of the primary key that supports range scans. — Option A is correct because in Cloud Spanner, the primary key order determines how data is physically sorted and stored. By making Timestamp the leading column of the primary key (e.g., (Timestamp, UserId)), range scans on Timestamp become efficient as Spanner can perform a contiguous scan of the sorted data. This directly addresses the slow queries that filter only by Timestamp range across all users.
What should I do if I get this PCDE question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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