The answer is a missing index on customer_id, as the Cloud Spanner Query Insights exhibit reveals high latency and excessive row scan counts for a query filtering on that column. This occurs because without an index, Cloud Spanner must perform a full table scan to locate matching rows, dramatically increasing read overhead and response time—a classic symptom of missing index detection with Query Insights. On the Google Professional Cloud Database Engineer exam, this scenario tests your ability to interpret Query Insights metrics and diagnose performance bottlenecks, often appearing as a graph showing high row scans alongside a filtered column. A common trap is assuming high latency stems from interleaved tables or stale data, but the row scan count directly points to an unindexed filter. Memory tip: “Scan high, index shy” — if row scans spike on a filtered column, the index is missing.
PCDE Monitor and optimize database performance Practice Question
This PCDE practice question tests your understanding of monitor and optimize database performance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
Refer to the exhibit.
```
Query: SELECT * FROM Orders WHERE customer_id = @c1 AND order_date > @d1
Total execution count: 1000
Average CPU time: 50ms
Average latency: 200ms
Rows scanned: 100000
Rows returned: 10
```
Based on the exhibit from Cloud Spanner Query Insights, what is the most likely performance issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit.
```
Query: SELECT * FROM Orders WHERE customer_id = @c1 AND order_date > @d1
Total execution count: 1000
Average CPU time: 50ms
Average latency: 200ms
Rows scanned: 100000
Rows returned: 10
```
A
High network latency
Why wrong: Network latency would increase latency but not the rows scanned metric.
B
Full table scan
Why wrong: Full table scan is the symptom, not the root cause. The root cause is missing index.
C
Inefficient join
Why wrong: The query involves only one table, so no join issue.
D
No index on customer_id
Missing index causes a full scan of the Orders table.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
No index on customer_id
Option D is correct because the exhibit shows a query with a filter on `customer_id` that is not indexed, forcing Cloud Spanner to perform a full table scan to find matching rows. This is the most likely performance issue, as indicated by high latency and high row scan counts in Query Insights, which directly points to a missing index on the filtered column.
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.
✗
High network latency
Why it's wrong here
Network latency would increase latency but not the rows scanned metric.
✗
Full table scan
Why it's wrong here
Full table scan is the symptom, not the root cause. The root cause is missing index.
✗
Inefficient join
Why it's wrong here
The query involves only one table, so no join issue.
✓
No index on customer_id
Why this is correct
Missing index causes a full scan of the Orders table.
Clue confirmation
The clue word "most likely" 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
Google Cloud often tests the distinction between a symptom (full table scan) and its root cause (missing index), tricking candidates into selecting the visible effect rather than the underlying configuration issue.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed storage engine where indexes are stored as separate interleaved tables; without an index on `customer_id`, the query must scan all splits of the base table, leading to high CPU and latency. In practice, this is a common pitfall when migrating from relational databases that automatically create indexes on foreign keys, as Spanner requires explicit index creation for filtered columns. The Query Insights 'Rows scanned' metric directly correlates with the number of rows read from storage, which spikes dramatically without an index.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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: No index on customer_id — Option D is correct because the exhibit shows a query with a filter on `customer_id` that is not indexed, forcing Cloud Spanner to perform a full table scan to find matching rows. This is the most likely performance issue, as indicated by high latency and high row scan counts in Query Insights, which directly points to a missing index on the filtered column.
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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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