The answer is that the tables are not interleaved, so parent and child rows may be in different splits, causing slow joins. Without interleaving, Cloud Spanner cannot guarantee colocation of related data, forcing distributed queries that coordinate across splits and degrade performance. This tests your understanding of schema design for join optimization on the Google Professional Cloud Database Engineer exam, where interleaved tables are a key strategy to avoid cross-split joins. A common trap is assuming a foreign key alone ensures row locality, but interleaving is required to physically store child rows adjacent to their parent. Remember the memory tip: “Interleave to retrieve, foreign key to relate”—the foreign key defines the relationship, but only interleaving makes the join fast by keeping data together.
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.
Exhibit
Refer to the exhibit.
-- Cloud Spanner DDL statement
CREATE TABLE Users (
UserId INT64 NOT NULL,
Name STRING(100) NOT NULL,
Email STRING(100) NOT NULL
) PRIMARY KEY (UserId);
CREATE TABLE Orders (
OrderId INT64 NOT NULL,
UserId INT64 NOT NULL,
OrderDate DATE NOT NULL,
Amount FLOAT64 NOT NULL,
CONSTRAINT FK_User FOREIGN KEY (UserId) REFERENCES Users (UserId)
) PRIMARY KEY (OrderId);
Refer to the exhibit. A developer creates these tables and notices that queries joining Users and Orders on UserId are slow. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Tables are not interleaved, so parent and child rows may be in different splits.
Option C is correct because without interleaving, parent and child rows may be stored on different splits, causing distributed joins. Option A is wrong because there is a foreign key. Option B is wrong because the primary key is on OrderId, not a composite key. Option D is wrong because the foreign key is defined correctly.
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.
✗
The primary key of Orders should include UserId as a prefix for co-location.
Why it's wrong here
For co-location, interleaving is needed; simply adding UserId to primary key is not enough if tables are not interleaved.
✗
The foreign key constraint is missing, causing full table scans.
Why it's wrong here
The foreign key exists; missing index on UserId in Orders might be an issue but not the primary reason.
✓
Tables are not interleaved, so parent and child rows may be in different splits.
Why this is correct
Interleaving is required to guarantee co-location. Without it, joins may be distributed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The foreign key reference should be on the parent table.
Why it's wrong here
Foreign keys are defined on the child table; this is correct.
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 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.
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.
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: Tables are not interleaved, so parent and child rows may be in different splits. — Option C is correct because without interleaving, parent and child rows may be stored on different splits, causing distributed joins. Option A is wrong because there is a foreign key. Option B is wrong because the primary key is on OrderId, not a composite key. Option D is wrong because the foreign key is defined correctly.
What should I do if I get this PCDE question wrong?
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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