- A
Use a secondary index on transactions.account_id.
Why wrong: While possible, a secondary index is less efficient than interleaving for this access pattern and adds write overhead.
- B
Ensure the primary key of transactions includes the account_id as the first part.
This is required for interleaved tables: the child's primary key must start with the parent's primary key.
- C
Define a foreign key constraint from transactions to accounts.
Why wrong: Cloud Spanner does not enforce foreign keys; they are informational only.
- D
Store transactions as a JSON array of repeating fields within the account record.
Why wrong: Denormalization limits query flexibility and indexing, and is not recommended for Spanner.
- E
Use an interleaved table hierarchy with accounts as parent and transactions as child.
Interleaved tables co-locate child rows with the parent row, optimizing queries that access transactions by account.
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.
A financial services company is designing a Cloud Spanner schema for a trading system. They have two main entities: 'accounts' and 'transactions'. Each account has many transactions, and queries almost always retrieve transactions for a specific account. Which TWO schema design strategies should they employ?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"always"Why it matters: Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
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
Ensure the primary key of transactions includes the account_id as the first part.
Option B is correct because Cloud Spanner distributes rows across splits based on the primary key prefix. By making `account_id` the first part of the transactions table primary key, all transactions for a given account are co-located, enabling efficient range scans and point lookups without cross-node shuffling.
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.
- ✗
Use a secondary index on transactions.account_id.
Why it's wrong here
While possible, a secondary index is less efficient than interleaving for this access pattern and adds write overhead.
- ✓
Ensure the primary key of transactions includes the account_id as the first part.
Why this is correct
This is required for interleaved tables: the child's primary key must start with the parent's primary key.
Clue confirmation
The clue word "always" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Define a foreign key constraint from transactions to accounts.
Why it's wrong here
Cloud Spanner does not enforce foreign keys; they are informational only.
- ✗
Store transactions as a JSON array of repeating fields within the account record.
Why it's wrong here
Denormalization limits query flexibility and indexing, and is not recommended for Spanner.
- ✓
Use an interleaved table hierarchy with accounts as parent and transactions as child.
Why this is correct
Interleaved tables co-locate child rows with the parent row, optimizing queries that access transactions by account.
Clue confirmation
The clue word "always" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that secondary indexes are the default solution for filtering, when in Cloud Spanner the primary key design and interleaving are the preferred strategies for performance and cost efficiency.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed, strongly consistent storage system where table data is split into tablets based on the primary key range. Interleaving tables (Option E) physically co-locates child rows with their parent row, reducing round trips and enabling atomic batch operations. This design is ideal for hierarchical 1:N relationships like accounts and transactions, as it ensures transactional isolation and low-latency queries for a specific account.
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.
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FAQ
Questions learners often ask
What does this PCDE question test?
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: Ensure the primary key of transactions includes the account_id as the first part. — Option B is correct because Cloud Spanner distributes rows across splits based on the primary key prefix. By making `account_id` the first part of the transactions table primary key, all transactions for a given account are co-located, enabling efficient range scans and point lookups without cross-node shuffling.
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: "always". Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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