PCDE Design and implement database schemas Practice Question
This PCDE practice question tests your understanding of design and implement database schemas. 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.
Your company runs a global e-commerce platform on Google Cloud Spanner. The database schema includes an 'Orders' table with primary key (OrderId, CustomerId) and an 'OrderItems' table with primary key (OrderId, CustomerId, ItemId), interleaved in parent Orders on delete cascade. During peak shopping hours, you notice that queries retrieving all items for a specific order are performing full table scans on the OrderItems table, leading to increased latency and higher CPU utilization. The queries use the OrderId as the filter condition. The database administrators have already checked that the query plans show table scans instead of using the interleaved index. You are tasked with resolving this performance issue. Which of the following actions should you take?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
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.
Remove CustomerId from the Orders primary key (making it just OrderId) and update OrderItems to have primary key (OrderId, ItemId), maintaining interleaving.
This allows efficient lookup using only OrderId and leverages interleaving.
B
Change the primary key of Orders to (OrderId, CustomerId) and update OrderItems accordingly.
Why wrong: This is the current state; no improvement.
C
Create a secondary index on OrderItems(OrderId).
Why wrong: This would help but does not fix the underlying schema inefficiency; interleaving should be used.
D
Increase the number of Spanner nodes to improve throughput.
Why wrong: More nodes do not fix the query efficiency; they only increase capacity.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Remove CustomerId from the Orders primary key (making it just OrderId) and update OrderItems to have primary key (OrderId, ItemId), maintaining interleaving.
Option A is correct because the interleaved index in Cloud Spanner requires that the parent table's primary key columns be a prefix of the child table's primary key. With the original schema, queries filtering only on OrderId cannot use the interleaved index because CustomerId is missing from the filter, forcing a full table scan. By removing CustomerId from the primary key of Orders and OrderItems, OrderId becomes the leading column, allowing the interleaved index to be used for efficient point lookups.
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.
✓
Remove CustomerId from the Orders primary key (making it just OrderId) and update OrderItems to have primary key (OrderId, ItemId), maintaining interleaving.
Why this is correct
This allows efficient lookup using only OrderId and leverages interleaving.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Change the primary key of Orders to (OrderId, CustomerId) and update OrderItems accordingly.
Why it's wrong here
This is the current state; no improvement.
✗
Create a secondary index on OrderItems(OrderId).
Why it's wrong here
This would help but does not fix the underlying schema inefficiency; interleaving should be used.
✗
Increase the number of Spanner nodes to improve throughput.
Why it's wrong here
More nodes do not fix the query efficiency; they only increase capacity.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that secondary indexes are the default fix for query performance issues, when in fact the schema design—specifically the primary key structure for interleaved tables—is the root cause and must be corrected first.
Detailed technical explanation
How to think about this question
Cloud Spanner interleaved tables store child rows physically adjacent to their parent row, enabling efficient joins and lookups when the parent key is a prefix of the child key. The interleaved index is automatically used for queries that include all leading columns of the primary key; missing any column forces a scan. In real-world scenarios, this design is critical for high-traffic e-commerce platforms where order-level queries must be fast, and schema changes like this are preferred over secondary indexes to avoid additional storage and maintenance costs.
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: Remove CustomerId from the Orders primary key (making it just OrderId) and update OrderItems to have primary key (OrderId, ItemId), maintaining interleaving. — Option A is correct because the interleaved index in Cloud Spanner requires that the parent table's primary key columns be a prefix of the child table's primary key. With the original schema, queries filtering only on OrderId cannot use the interleaved index because CustomerId is missing from the filter, forcing a full table scan. By removing CustomerId from the primary key of Orders and OrderItems, OrderId becomes the leading column, allowing the interleaved index to be used for efficient point lookups.
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: "primary". Asks for the main purpose or function, not a secondary benefit. Eliminate answers that describe side-effects or partial functions.
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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