PCDOE Optimizing service performance Practice Question
This PCDOE practice question tests your understanding of optimizing service performance. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 team uses Cloud Spanner for a global application. Query performance degrades as data grows. They notice that most queries filter on a column 'customer_id' but the primary key is a UUID. What is the best approach to optimize performance?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Reorder the primary key to start with customer_id
In Cloud Spanner, the primary key determines the physical ordering of rows on storage tablets. When queries filter on `customer_id` but the primary key starts with a UUID, Spanner must perform a full table scan because the filter cannot leverage the key order. Reordering the primary key to start with `customer_id` allows Spanner to use efficient key-range scans, dramatically reducing the number of rows read per query.
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.
✗
Enable query optimization hints
Why it's wrong here
Hints provide query-level guidance but do not address the underlying data distribution issue.
✓
Reorder the primary key to start with customer_id
Why this is correct
Putting customer_id first in the primary key distributes writes and optimizes queries on that column.
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.
✗
Use interleaved tables
Why it's wrong here
Interleaved tables are for hierarchical relationships, not for optimizing query performance on a non-key column.
✗
Create a secondary index on customer_id
Why it's wrong here
A secondary index improves reads but does not solve write hot spotting.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that secondary indexes are always the best solution for query performance, when in fact reordering the primary key to match the most common query filter pattern is more efficient because it avoids the extra index lookup and write overhead.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed, strongly consistent storage architecture where rows are sorted by primary key within each split (tablet). By starting the primary key with `customer_id`, queries with equality or range filters on that column can be served from a contiguous range of tablets, minimizing cross-node communication and reducing latency. In contrast, a secondary index requires an extra lookup from the index table to the base table, which can add a round-trip per row in worst-case scenarios, especially under high contention.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Optimizing service performance — This question tests Optimizing service performance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Reorder the primary key to start with customer_id — In Cloud Spanner, the primary key determines the physical ordering of rows on storage tablets. When queries filter on `customer_id` but the primary key starts with a UUID, Spanner must perform a full table scan because the filter cannot leverage the key order. Reordering the primary key to start with `customer_id` allows Spanner to use efficient key-range scans, dramatically reducing the number of rows read per query.
What should I do if I get this PCDOE 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", "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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Question Discussion
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