The answer is a missing index on the status column, as this is the most likely cause of high execution time when diagnosing slow queries in Cloud SQL. When a query filters on a column like status without an index, the database engine must perform a full table scan, reading every row to find matches—a costly operation that scales linearly with table size. Adding an index on the filtered column allows the engine to seek directly to relevant rows, drastically reducing I/O and execution time. On the Google Professional Cloud Database Engineer exam, this scenario tests your ability to interpret execution plans and identify index-related performance bottlenecks; a common trap is to blame CPU or memory limits when the real culprit is a missing index. Remember the mnemonic: “No index, full scan—slow as molasses in a pan.”
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.
Exhibit
Refer to the exhibit. Cloud Spanner query statistics:
Query: SELECT customer_id, order_total FROM orders WHERE status = 'PENDING'
Execution time: 4.2 seconds
Rows returned: 500
Rows scanned: 5,000,000
Nodes: 2
Execution plan: Full table scan
What is the most likely cause of the high execution time?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Missing index on status column
The execution plan shows a full table scan. Since the query filters on status, an index on the status column would avoid the scan and reduce execution time. Options A, B, D, E are not the primary cause.
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.
✗
The WHERE clause uses a string comparison
Why it's wrong here
String comparison is not inherently slow; the lack of an index is the problem.
✓
Missing index on status column
Why this is correct
An index on status would allow efficient row retrieval instead of a full table scan.
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.
✗
The query selects two columns causing inefficiency
Why it's wrong here
Selecting two columns is not inefficient; the scan is the issue.
✗
The instance has insufficient nodes
Why it's wrong here
Node count of 2 is generally sufficient; the full table scan is the bottleneck, not node count.
✗
The query does not filter on primary key
Why it's wrong here
Filtering on primary key is not required; the issue is the missing index on the status column.
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 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 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.
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: Missing index on status column — The execution plan shows a full table scan. Since the query filters on status, an index on the status column would avoid the scan and reduce execution time. Options A, B, D, E are not the primary cause.
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: "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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Question Discussion
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