- A
Disable automatic indexing and create custom composite indexes only for queries that require them.
Automatic indexing causes every property to be indexed, resulting in excessive write operations. Custom indexes reduce write amplification.
- B
Implement a data retention policy to delete old tenant data.
Why wrong: Deleting old data reduces storage but does not improve current write throughput degradation.
- C
Request a throughput increase from Google Cloud support.
Why wrong: Firestore automatically scales; there is no manual throughput increase except by using better index design.
- D
Shard tenants into separate Firestore databases to distribute the write load.
Why wrong: Sharding databases may help but does not fix the root cause of index-related write amplification.
Quick Answer
The answer is to disable automatic indexing and create custom composite indexes only for queries that require them. This is correct because Firestore Datastore mode’s automatic indexing creates an index for every property, leading to severe write amplification—each write must update all those indexes. In a multi-tenant SaaS app with separate namespaces, the number of indexes scales linearly with tenants, so write throughput degrades as latency increases even when write volume stays within limits. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of how automatic indexing impacts write performance in Datastore mode, a common trap where engineers overlook index overhead when scaling tenants. Remember the key insight: automatic indexing is a convenience that becomes a bottleneck under high write loads with many entities. A useful memory tip is “index every property, pay for every write”—custom indexes trade setup effort for throughput.
PCDE Plan and manage database infrastructure Practice Question
This PCDE practice question tests your understanding of plan and manage database infrastructure. 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 company uses Cloud Firestore in Datastore mode for a multi-tenant SaaS application. Each tenant has a separate namespace. The application has grown rapidly, and the database engineer notices that write throughput is degrading. Monitoring shows that the number of writes per second is high but within Firestore limits. However, the latency for writes is increasing linearly with the number of tenants. The engineer suspects that index management is causing the problem. The current schema uses automatic indexes for all properties. What is the best corrective action?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"best"Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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
Disable automatic indexing and create custom composite indexes only for queries that require them.
The correct answer is A because automatic indexing in Firestore in Datastore mode creates an index for every property, which causes write amplification as each write must update all relevant indexes. With many tenants in separate namespaces, the number of indexes grows linearly with tenants, increasing write latency. Disabling automatic indexing and creating custom composite indexes only for needed queries reduces the index write overhead, restoring write throughput.
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.
- ✓
Disable automatic indexing and create custom composite indexes only for queries that require them.
Why this is correct
Automatic indexing causes every property to be indexed, resulting in excessive write operations. Custom indexes reduce write amplification.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Implement a data retention policy to delete old tenant data.
Why it's wrong here
Deleting old data reduces storage but does not improve current write throughput degradation.
- ✗
Request a throughput increase from Google Cloud support.
Why it's wrong here
Firestore automatically scales; there is no manual throughput increase except by using better index design.
- ✗
Shard tenants into separate Firestore databases to distribute the write load.
Why it's wrong here
Sharding databases may help but does not fix the root cause of index-related write amplification.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume the issue is throughput capacity (Option C) or data volume (Option B), rather than recognizing that automatic indexing creates a write amplification problem that scales with schema complexity and tenant count.
Detailed technical explanation
How to think about this question
Firestore in Datastore mode automatically creates a single-property index for each property in an entity, and each write must update all such indexes. In a multi-tenant setup with separate namespaces, the number of indexes scales with the number of distinct properties across tenants, causing write latency to increase linearly. Custom composite indexes, when used selectively, avoid this overhead by indexing only the properties used in queries, reducing the number of index entries per write.
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
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCDE question test?
Plan and manage database infrastructure — This question tests Plan and manage database infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Disable automatic indexing and create custom composite indexes only for queries that require them. — The correct answer is A because automatic indexing in Firestore in Datastore mode creates an index for every property, which causes write amplification as each write must update all relevant indexes. With many tenants in separate namespaces, the number of indexes grows linearly with tenants, increasing write latency. Disabling automatic indexing and creating custom composite indexes only for needed queries reduces the index write overhead, restoring write throughput.
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: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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