- A
A composite index on (conversationId, timestamp, __name__) descending
This index covers the query with filtering and ordering, enabling efficient retrieval.
- B
A single-field index on timestamp
Why wrong: Does not filter by conversationId, would require scanning all messages.
- C
An index on conversationId only
Why wrong: Filters by conversation but cannot order by timestamp efficiently; requires in-memory sort.
- D
A composite index on (senderId, timestamp)
Why wrong: Not relevant; query does not filter by senderId.
Question 184 of 1,000
PCDE Design and implement database schemas Practice Question
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.
You are designing a Firestore database for a chat application. Documents will store messages with fields: senderId, messageText, timestamp, conversationId. To efficiently retrieve the most recent 50 messages in a conversation, which index should you create?
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
✓
A composite index on (conversationId, timestamp, __name__) descending
Option A is correct because to efficiently retrieve the most recent 50 messages for a specific conversation, you need a composite index on (conversationId, timestamp, __name__) with descending order. This allows Firestore to perform a range query on conversationId and then order by timestamp descending, using the index to skip scanning irrelevant documents. The __name__ field is included to ensure the index is covering and to handle document name ordering ties, which is required for consistent pagination with descending 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.
- ✓
A composite index on (conversationId, timestamp, __name__) descending
Why this is correct
This index covers the query with filtering and ordering, enabling efficient retrieval.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
A single-field index on timestamp
Why it's wrong here
Does not filter by conversationId, would require scanning all messages.
- ✗
An index on conversationId only
Why it's wrong here
Filters by conversation but cannot order by timestamp efficiently; requires in-memory sort.
- ✗
A composite index on (senderId, timestamp)
Why it's wrong here
Not relevant; query does not filter by senderId.
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common misconception is that a single-field index on the ordering field (timestamp) is sufficient for filtered and ordered queries, but Firestore requires a composite index that includes all equality filter fields before the order field to avoid a full collection scan. Additionally, including __name__ in the index ensures proper handling of document name sorting for consistent pagination with descending order.
Detailed technical explanation
How to think about this question
Firestore composite indexes are stored as sorted lists of index entries; when you query with an order by clause, the index must have the order field as the last equality field in the index definition. Including __name__ (the document ID) in the index ensures that the index is fully covering and that Firestore can handle cases where multiple documents have the same timestamp, as document IDs are unique and provide a deterministic tiebreaker for ordering. In practice, without __name__, descending queries on timestamp can produce inconsistent pagination cursors if timestamps are not unique.
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
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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: A composite index on (conversationId, timestamp, __name__) descending — Option A is correct because to efficiently retrieve the most recent 50 messages for a specific conversation, you need a composite index on (conversationId, timestamp, __name__) with descending order. This allows Firestore to perform a range query on conversationId and then order by timestamp descending, using the index to skip scanning irrelevant documents. The __name__ field is included to ensure the index is covering and to handle document name ordering ties, which is required for consistent pagination with descending order.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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