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.
gcloud spanner databases ddl update my-database --ddl='CREATE TABLE Customers (CustomerId INT64 NOT NULL, Name STRING(100), Email STRING(100)) PRIMARY KEY (CustomerId); CREATE TABLE Orders (OrderId INT64 NOT NULL, CustomerId INT64 NOT NULL, OrderDate DATE, Amount FLOAT64) PRIMARY KEY (OrderId, CustomerId), INTERLEAVE IN PARENT Customers ON DELETE CASCADE;'
A team executed the above DDL to create interleaved tables in Cloud Spanner. They need to query all orders for a specific customer. Which query will be most efficient?
Exhibit
Refer to the exhibit.
gcloud spanner databases ddl update my-database --ddl='CREATE TABLE Customers (CustomerId INT64 NOT NULL, Name STRING(100), Email STRING(100)) PRIMARY KEY (CustomerId); CREATE TABLE Orders (OrderId INT64 NOT NULL, CustomerId INT64 NOT NULL, OrderDate DATE, Amount FLOAT64) PRIMARY KEY (OrderId, CustomerId), INTERLEAVE IN PARENT Customers ON DELETE CASCADE;'
A
SELECT * FROM Orders WHERE CustomerId = 1234 AND OrderDate = '2023-01-01';
Why wrong: While this uses CustomerId, the additional condition does not improve efficiency beyond option A, but it is still efficient.
B
SELECT * FROM Customers JOIN Orders ON Customers.CustomerId = Orders.CustomerId WHERE Customers.CustomerId = 1234;
Why wrong: A join is unnecessary because interleaving already links the tables; it will still work but is more complex.
C
SELECT * FROM Orders WHERE CustomerId = 1234;
Interleaving colocates all orders for a customer, making this query very efficient.
D
SELECT * FROM Orders WHERE OrderId = 5678;
Why wrong: Without filtering on CustomerId, this query may not benefit from interleaving and could be slower.
The answer is SELECT * FROM Orders WHERE CustomerId = 1234. This query is most efficient because Cloud Spanner interleaved tables physically colocate child rows with their parent row on the same split, so filtering by the parent key CustomerId allows Spanner to perform a direct, single-split lookup rather than scattering reads across nodes. On the Google Professional Cloud Database Engineer exam, this tests your understanding of interleaved table query optimization and how schema design drives query performance—a common trap is assuming a join is needed when interleaving already provides the relationship, or filtering only by OrderId, which forces a full table scan. Remember the key insight: interleaving is about locality, not just logical hierarchy. For a quick memory tip, think “parent key first, child rows local”—always lead your WHERE clause with the interleaved parent column to avoid cross-split scans.
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
✓
SELECT * FROM Orders WHERE CustomerId = 1234;
Option C is correct because in Cloud Spanner, interleaved tables store child rows physically adjacent to their parent row within the same split. Querying Orders directly on the interleaved key (CustomerId) allows Spanner to perform a local index scan within the parent row's split, avoiding a distributed cross-table join. This leverages the interleaved table's physical clustering for the most efficient retrieval.
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.
✗
SELECT * FROM Orders WHERE CustomerId = 1234 AND OrderDate = '2023-01-01';
Why it's wrong here
While this uses CustomerId, the additional condition does not improve efficiency beyond option A, but it is still efficient.
✗
SELECT * FROM Customers JOIN Orders ON Customers.CustomerId = Orders.CustomerId WHERE Customers.CustomerId = 1234;
Why it's wrong here
A join is unnecessary because interleaving already links the tables; it will still work but is more complex.
✓
SELECT * FROM Orders WHERE CustomerId = 1234;
Why this is correct
Interleaving colocates all orders for a customer, making this query very efficient.
Related concept
Read the scenario before looking for a memorised answer.
✗
SELECT * FROM Orders WHERE OrderId = 5678;
Why it's wrong here
Without filtering on CustomerId, this query may not benefit from interleaving and could be slower.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that an explicit JOIN is required for interleaved tables, but the correct approach is to query the child table directly using the parent key, as the interleaved structure already enforces the relationship without a join.
Detailed technical explanation
How to think about this question
Cloud Spanner's interleaved tables store child rows in the same split as the parent row, co-located by the parent key prefix. When querying by the parent key (CustomerId), Spanner can perform a range scan within that split without consulting other nodes, achieving low-latency reads. This design is critical for hierarchical data access patterns, such as orders per customer, where the parent key is always specified in the WHERE clause.
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 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 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: SELECT * FROM Orders WHERE CustomerId = 1234; — Option C is correct because in Cloud Spanner, interleaved tables store child rows physically adjacent to their parent row within the same split. Querying Orders directly on the interleaved key (CustomerId) allows Spanner to perform a local index scan within the parent row's split, avoiding a distributed cross-table join. This leverages the interleaved table's physical clustering for the most efficient retrieval.
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.
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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