- A
(CustomerId, OrderId)
Why wrong: OrderId is monotonically increasing but does not order by date.
- B
Add a commit timestamp column as part of the primary key
Why wrong: Commit timestamps are not suitable for primary keys due to potential hotspotting.
- C
No change; use a secondary index on OrderDate
Why wrong: Secondary indexes add write overhead and are not as efficient as primary key ordering.
- D
(CustomerId, OrderDate DESC)
Descending order stores newest first, enabling efficient limit queries.
Question 69 of 1,000
PCDE Interleaved Tables 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. A key principle to apply: interleaved Tables. 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 Cloud Spanner database has a parent table 'Customers' and a child table 'Orders' interleaved on CustomerId. The most common query retrieves the last 10 orders for a given customer. How should the primary key of Orders be defined for optimal performance?
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.
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
✓
(CustomerId, OrderDate DESC)
Option D is correct because defining the primary key as (CustomerId, OrderDate DESC) leverages Cloud Spanner's interleaved table structure to physically co-locate rows for the same customer, and the descending order on OrderDate allows the most common query (retrieving the last 10 orders) to be served as a sequential scan of the most recent rows without sorting or a full table scan.
Key principle: Interleaved Tables
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
(CustomerId, OrderId)
Why it's wrong here
OrderId is monotonically increasing but does not order by date.
- ✗
Add a commit timestamp column as part of the primary key
Why it's wrong here
Commit timestamps are not suitable for primary keys due to potential hotspotting.
- ✗
No change; use a secondary index on OrderDate
Why it's wrong here
Secondary indexes add write overhead and are not as efficient as primary key ordering.
- ✓
(CustomerId, OrderDate DESC)
Why this is correct
Descending order stores newest first, enabling efficient limit queries.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Interleaved Tables
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common trap in Google Cloud exams is assuming that a secondary index on OrderDate is sufficient for ordered retrieval. However, without using the primary key ordering in an interleaved table, the query cannot exploit physical co-location and will require an extra index lookup and sort, which is suboptimal.
Detailed technical explanation
How to think about this question
Cloud Spanner interleaved tables store child rows physically adjacent to their parent row, so with (CustomerId, OrderDate DESC) as the primary key, the last 10 orders for a customer are stored consecutively in the most recent portion of the key range. This allows Spanner to perform a range scan on the primary key with a LIMIT 10, avoiding any sorting or index maintenance overhead. In real-world scenarios, this design is critical for high-throughput OLTP workloads where per-customer order history queries must complete in single-digit milliseconds.
KKey Concepts to Remember
- Interleaved Tables
- Primary Key Ordering
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
Interleaved Tables
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. Interleaved Tables 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.
Review interleaved Tables, then practise related PCDE questions on the same topic to reinforce the concept.
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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 — Interleaved Tables.
What is the correct answer to this question?
The correct answer is: (CustomerId, OrderDate DESC) — Option D is correct because defining the primary key as (CustomerId, OrderDate DESC) leverages Cloud Spanner's interleaved table structure to physically co-locate rows for the same customer, and the descending order on OrderDate allows the most common query (retrieving the last 10 orders) to be served as a sequential scan of the most recent rows without sorting or a full table scan.
What should I do if I get this PCDE question wrong?
Review interleaved Tables, then practise related PCDE questions on the same topic to reinforce the concept.
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?
Interleaved Tables
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Last reviewed: Jul 4, 2026
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