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Question 854 of 1,000

Design and implement database schemas →hardMultiple ChoiceObjective-mapped

Cloud Spanner Secondary Index Hotspotting: Hash Prefix Solution

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.

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.

Full breakdown with real-world context →

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.”

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 hotspotting on the secondary index. Option B (migrating to Bigtable) introduces operational complexity and does not leverage Spanner's existing capabilities. Option C (changing the base table's primary key to start with campaign_id) would not fix the index hotspot because the secondary index on (campaign_id, timestamp) would still concentrate writes on a single split due to low cardinality of campaign_id. Option D (creating a separate aggregation table) adds complexity and would require additional mechanisms to keep it updated, which can be error-prone and may not solve the underlying index hotspot.

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.

✓
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
Read the scenario before looking for a memorised answer.
✗
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: 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 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.

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.

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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: 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 hotspotting on the secondary index. Option B (migrating to Bigtable) introduces operational complexity and does not leverage Spanner's existing capabilities. Option C (changing the base table's primary key to start with campaign_id) would not fix the index hotspot because the secondary index on (campaign_id, timestamp) would still concentrate writes on a single split due to low cardinality of campaign_id. Option D (creating a separate aggregation table) adds complexity and would require additional mechanisms to keep it updated, which can be error-prone and may not solve the underlying index hotspot.

What should I do if I get this PCDE question wrong?

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?

Read the scenario before looking for a memorised answer.

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