- A
Add a synthetic field that combines the Boolean with a timestamp for range queries.
Why wrong: A synthetic field might help but still needs an appropriate composite index.
- B
Create a composite index that includes the Boolean field and the query ordering field.
A composite index tailored to the query pattern improves performance and avoids full collection scans.
- C
Denormalize the Boolean field into separate subcollections.
Why wrong: This changes the schema structure and may complicate queries.
- D
Rely on the automatic single-field index already created.
Why wrong: Automatic indexing exists but for Boolean fields it may not be selective enough; composite index is better.
Question 358 of 1,000
Firestore Composite Index for Boolean Field Queries
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.
A Cloud Firestore database stores documents for a mobile app. The app frequently queries for documents where a specific Boolean field is true. The field is not part of the collection group index. What should the developer do to improve query performance?
Quick Answer
The answer is to create a composite index that includes the Boolean field and the query ordering field. This is correct because Firestore’s automatic single-field indexes on a Boolean field only support simple equality filters; when you query for documents where a Boolean field is true and also apply an ordering or additional filter, the query engine cannot efficiently satisfy both conditions without a composite index that pairs the Boolean field with the ordering field. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of how Firestore’s index merging works and the common pitfall of assuming single-field indexes are sufficient for filtered, ordered queries. A frequent trap is thinking denormalization or adding extra fields will solve performance issues, but without a proper composite index, the query will still scan all documents. Remember the memory tip: “Bool plus order, composite border” — whenever a Boolean filter meets an order clause, you need a composite index to cross that performance border.
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
✓
Create a composite index that includes the Boolean field and the query ordering field.
Option B is correct because Cloud Firestore requires a composite index to efficiently query on a Boolean field combined with an ordering field. Without this index, the query would perform a full collection scan, leading to poor performance. Creating a composite index that includes the Boolean field and the query ordering field allows Firestore to use the index to directly locate matching documents, avoiding expensive sequential scans.
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.
- ✗
Add a synthetic field that combines the Boolean with a timestamp for range queries.
Why it's wrong here
A synthetic field might help but still needs an appropriate composite index.
- ✓
Create a composite index that includes the Boolean field and the query ordering field.
Why this is correct
A composite index tailored to the query pattern improves performance and avoids full collection scans.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Denormalize the Boolean field into separate subcollections.
Why it's wrong here
This changes the schema structure and may complicate queries.
- ✗
Rely on the automatic single-field index already created.
Why it's wrong here
Automatic indexing exists but for Boolean fields it may not be selective enough; composite index is better.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Candidates often assume that automatic single-field indexes are sufficient for all queries, but in Firestore, queries with both a filter and an order-by clause require a composite index, even if the filter is on a simple Boolean field.
Detailed technical explanation
How to think about this question
Under the hood, Firestore uses indexes to map indexed values to document locations. A composite index stores entries sorted by the first field (e.g., the Boolean field) and then by the second field (e.g., timestamp), enabling efficient filtering and ordering in a single index scan. In real-world scenarios, failing to create a composite index for queries with equality filters on one field and order on another can cause Firestore to fall back to a full table scan, which becomes increasingly slow as the collection grows, especially with millions of documents.
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: Create a composite index that includes the Boolean field and the query ordering field. — Option B is correct because Cloud Firestore requires a composite index to efficiently query on a Boolean field combined with an ordering field. Without this index, the query would perform a full collection scan, leading to poor performance. Creating a composite index that includes the Boolean field and the query ordering field allows Firestore to use the index to directly locate matching documents, avoiding expensive sequential scans.
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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