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.
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);
A
The primary key of Orders should include UserId as a prefix for co-location.
Why wrong: For co-location, interleaving is needed; simply adding UserId to primary key is not enough if tables are not interleaved.
B
The foreign key constraint is missing, causing full table scans.
Why wrong: The foreign key exists; missing index on UserId in Orders might be an issue but not the primary reason.
C
Tables are not interleaved, so parent and child rows may be in different splits.
Interleaving is required to guarantee co-location. Without it, joins may be distributed.
D
The foreign key reference should be on the parent table.
Why wrong: Foreign keys are defined on the child table; this is correct.
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.
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
✓
Tables are not interleaved, so parent and child rows may be in different splits.
In Google Cloud Spanner, interleaving tables physically co-locates parent and child rows in the same tablet split, reducing cross-node lookups. Without interleaving, rows from Users and Orders may reside on different servers, causing distributed queries that are slower due to network round trips. Option C correctly identifies this as the most likely cause of slow joins.
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
Google often tests the misconception that foreign keys or primary key ordering alone solve performance issues, when in Cloud Spanner the key optimization is interleaving for co-location.
Detailed technical explanation
How to think about this question
CockroachDB's interleaving stores child rows directly adjacent to their parent row within the same key-value range, using the parent's primary key as a prefix. This eliminates the need for distributed queries across nodes when joining on the interleaved key. Without interleaving, a join on UserId may require scanning multiple ranges and performing network hops, which is significantly slower in a distributed database.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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.
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. — In Google Cloud Spanner, interleaving tables physically co-locates parent and child rows in the same tablet split, reducing cross-node lookups. Without interleaving, rows from Users and Orders may reside on different servers, causing distributed queries that are slower due to network round trips. Option C correctly identifies this as the most likely cause of slow joins.
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: "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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Question Discussion
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