- A
Modify the secondary index to include a hash prefix (e.g., use 'hash(campaign_id)' as the first column of the index).
A hash prefix distributes index writes evenly across splits, preventing hotspotting.
- B
Migrate the ad_impressions table to Cloud Bigtable with row key 'campaign_id#timestamp'.
Why wrong: Migrating databases is a major architectural change, not a schema redesign.
- C
Change the primary key of the base table to include campaign_id as the first column.
Why wrong: This changes data distribution but does not address the index hotspotting; the index may still be needed.
- D
Create a separate table that stores per-campaign aggregations, updated in real time.
Why wrong: This adds maintenance overhead and still may require an index or distribution strategy.
Quick Answer
The answer is to modify the secondary index to include a hash prefix, such as using hash(campaign_id) as the first column of the index. This directly solves Cloud Spanner index hotspotting with low cardinality because a low-cardinality column like campaign_id causes all writes for the same campaign to target a single index split, creating a bottleneck. By hashing the campaign_id, you distribute those writes across multiple splits, eliminating the hotspot while still supporting the campaign aggregation queries via the full index key. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of split distribution and the trade-off between query performance and write scalability—a common trap is to mistakenly change the primary key, which only affects base table distribution, not the secondary index. Remember the memory tip: “Hash the hot key to spread the load; low cardinality needs a prefix to explode.”
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.
An online advertising platform uses Cloud Spanner for ad impression tracking. The table 'ad_impressions' has a primary key (ad_id, timestamp). The table receives millions of writes per minute. A secondary index on (campaign_id, timestamp) was created to support queries that sum impressions per campaign. During high traffic, the team notices increased write latency and hotspotting on the index (the campaign_id has low cardinality, causing all writes to a campaign to hit the same index split). They need to redesign the schema to avoid hotspotting on the index while still supporting the campaign aggregation queries. What is the best solution?
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.
Clue:
"primary"Why it matters: Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
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
Modify the secondary index to include a hash prefix (e.g., use 'hash(campaign_id)' as the first column of the index).
Option A is correct. Adding a hash prefix to the index key (e.g., using a hash of campaign_id as the leading column) distributes index writes across multiple splits, eliminating the hotspot. Option B (changing primary key) would affect the base table distribution but not necessarily the index. Option C (separate table) adds complexity and still may have indexing issues. Option D (Bigtable) is a different database.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Modify the secondary index to include a hash prefix (e.g., use 'hash(campaign_id)' as the first column of the index).
Why this is correct
A hash prefix distributes index writes evenly across splits, preventing hotspotting.
Clue confirmation
The clue words "best", "primary" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
Migrate the ad_impressions table to Cloud Bigtable with row key 'campaign_id#timestamp'.
Why it's wrong here
Migrating databases is a major architectural change, not a schema redesign.
- ✗
Change the primary key of the base table to include campaign_id as the first column.
Why it's wrong here
This changes data distribution but does not address the index hotspotting; the index may still be needed.
- ✗
Create a separate table that stores per-campaign aggregations, updated in real time.
Why it's wrong here
This adds maintenance overhead and still may require an index or distribution strategy.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PCDE NAT questions on configuration and troubleshooting.
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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 — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: Modify the secondary index to include a hash prefix (e.g., use 'hash(campaign_id)' as the first column of the index). — Option A is correct. Adding a hash prefix to the index key (e.g., using a hash of campaign_id as the leading column) distributes index writes across multiple splits, eliminating the hotspot. Option B (changing primary key) would affect the base table distribution but not necessarily the index. Option C (separate table) adds complexity and still may have indexing issues. Option D (Bigtable) is a different database.
What should I do if I get this PCDE question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related PCDE NAT questions on configuration and troubleshooting.
Are there clue words in this question I should notice?
Yes — watch for: "best", "primary". 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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 24, 2026
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