The answer is that the composite index exists but the timestamp field is indexed in ascending order instead of descending. When a Firestore composite index on (status, timestamp) uses ascending order for timestamp, a query with `order by timestamp desc` cannot efficiently leverage that index; Firestore scans the index in the wrong direction, and because the index is not reversed, it may fail to locate matching documents, returning empty results even when active documents exist. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of how composite index order direction directly impacts query results—a common trap is assuming any index on the fields suffices, but the sort order must exactly match the query’s `order by` clause. Remember the memory tip: “Index order must mirror query order—asc for asc, desc for desc, or your results vanish.”
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.
A Firestore database has a collection with a composite index on (status, timestamp desc). The query `where status == 'active' order by timestamp desc limit 50` is returning empty results even though there are active documents. What could be the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The composite index exists but the order of timestamp is asc instead of desc.
If the composite index has timestamp in ascending order, the order by desc cannot use the index efficiently, leading to empty results due to scanning in wrong order.
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 status field is not indexed.
Why it's wrong here
Status is part of the composite index, so it is indexed.
✓
The composite index exists but the order of timestamp is asc instead of desc.
Why this is correct
The query requires descending order; using an ascending index can cause the query to miss documents.
Related concept
Read the scenario before looking for a memorised answer.
✗
The documents have 'active' status but timestamp field is missing.
Why it's wrong here
Missing timestamp would cause null values, but that would likely return documents with null timestamp, not empty results.
✗
The query requires an index on timestamp only.
Why it's wrong here
A single-field index on timestamp cannot satisfy both equality on status and order by timestamp.
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: The composite index exists but the order of timestamp is asc instead of desc. — If the composite index has timestamp in ascending order, the order by desc cannot use the index efficiently, leading to empty results due to scanning in wrong order.
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.
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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