- A
Add a secondary index on `user_id` to help narrow down the search.
Why wrong: Indexing `user_id` does not help because the query does not filter by `user_id`.
- B
Create a secondary index on `transaction_id` to enable efficient key-based lookups.
A secondary index on `transaction_id` provides a direct lookup path, reducing latency.
- C
Use a Spanner query hint to force a specific index scan.
Why wrong: Without an appropriate index, a hint cannot improve performance; the index must exist first.
- D
Change the primary key to `(transaction_id, user_id)` to enable direct access by transaction_id.
Why wrong: Changing the primary key of a large table is a disruptive operation and may affect interleaving with the parent table. Not recommended without careful planning.
Quick Answer
The answer is to create a secondary index on `transaction_id` to restore point-read performance. This is correct because Cloud Spanner’s primary key structure `(user_id, transaction_id)` requires the leading key column for efficient split routing; without `user_id`, queries must scatter across splits or perform a full scan, causing latency spikes from under 10ms to over 100ms. A secondary index on `transaction_id` provides a separate, ordered key structure that enables Spanner to perform a direct key-based lookup to the exact split, bypassing the full table scan and restoring sub-10ms reads. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of Spanner’s key-based split architecture and the critical role of leading key columns in point-read optimization—a common trap is assuming CPU or contention is the issue when the real problem is missing index coverage. Memory tip: “Lead with the key, or pay the latency fee”—always ensure your query’s filter column matches the primary key’s leading column, or create a secondary index to act as that lead.
PCDE Monitor and optimize database performance Practice Question
This PCDE practice question tests your understanding of monitor and optimize database performance. 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.
You are managing a Spanner instance for a global financial application. The database has a table `transactions` with columns `transaction_id` (INT64), `user_id` (INT64), `amount` (FLOAT64), `timestamp` (TIMESTAMP), and `region` (STRING). The table is interleaved with a parent table `users`. Recently, you observed that point-read queries by `transaction_id` are taking over 100ms on average, whereas they used to take under 10ms. The instance CPU utilization is below 40%, and there are no contention issues. The `transactions` table has a primary key `(user_id, transaction_id)`. Queries filter on `transaction_id` only, without specifying `user_id`. Which optimization should you implement to improve point-read latency?
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
Create a secondary index on `transaction_id` to enable efficient key-based lookups.
Point-read queries by `transaction_id` are slow because the primary key is `(user_id, transaction_id)`, so without `user_id`, Spanner cannot directly locate the split (tablet) and must perform a full table scan or a less efficient lookup. Creating a secondary index on `transaction_id` allows Spanner to use that index for key-based lookups, reducing latency to under 10ms by enabling direct access to the specific split via the index's key.
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.
- ✗
Add a secondary index on `user_id` to help narrow down the search.
Why it's wrong here
Indexing `user_id` does not help because the query does not filter by `user_id`.
- ✓
Create a secondary index on `transaction_id` to enable efficient key-based lookups.
Why this is correct
A secondary index on `transaction_id` provides a direct lookup path, reducing latency.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Use a Spanner query hint to force a specific index scan.
Why it's wrong here
Without an appropriate index, a hint cannot improve performance; the index must exist first.
- ✗
Change the primary key to `(transaction_id, user_id)` to enable direct access by transaction_id.
Why it's wrong here
Changing the primary key of a large table is a disruptive operation and may affect interleaving with the parent table. Not recommended without careful planning.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Google Cloud often tests the misconception that changing the primary key is the only way to optimize queries that don't use the full primary key, but in Spanner, secondary indexes are the correct and efficient solution without disrupting existing interleaved table relationships.
Detailed technical explanation
How to think about this question
Spanner uses a distributed storage architecture where data is split into tablets based on the primary key prefix. Without the leading `user_id` in the query, Spanner cannot prune splits and must scatter the request across all tablets, causing high latency. A secondary index on `transaction_id` creates a separate index table with its own split distribution, allowing Spanner to perform a point lookup directly to the correct split using the index's key, bypassing the full table scan.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this PCDE question test?
Monitor and optimize database performance — This question tests Monitor and optimize database performance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Create a secondary index on `transaction_id` to enable efficient key-based lookups. — Point-read queries by `transaction_id` are slow because the primary key is `(user_id, transaction_id)`, so without `user_id`, Spanner cannot directly locate the split (tablet) and must perform a full table scan or a less efficient lookup. Creating a secondary index on `transaction_id` allows Spanner to use that index for key-based lookups, reducing latency to under 10ms by enabling direct access to the specific split via the index's key.
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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Last reviewed: Jun 30, 2026
This PCDE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PCDE exam.
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