The answer is to add a secondary index on CustomerID. This is correct because Cloud Spanner requires the full primary key—in this case, (CustomerID, OrderDate)—for a direct point read; without the OrderDate component, Spanner must perform a full table scan or an inefficient range scan, causing high latency. A secondary index on CustomerID alone enables Spanner to first locate the exact row via an index scan, then execute a fast point read, dramatically improving performance for queries filtered only by CustomerID. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of how Spanner’s distributed architecture handles key-based lookups versus index scans, and a common trap is assuming a partial primary key is sufficient for efficient point reads. Remember the memory tip: “Full key for direct hit; index for partial fit.”
PCDE Monitor and optimize database performance Practice Question
This PCDE practice question tests your understanding of monitor and optimize database performance. 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 Cloud Spanner database is experiencing high latency for point reads. The table has a primary key of (CustomerID, OrderDate). Most reads are by CustomerID only. What should the engineer do?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add a secondary index on CustomerID.
Point reads by CustomerID only are inefficient on the primary key (CustomerID, OrderDate) because Cloud Spanner requires the full primary key for direct lookup. Adding a secondary index on CustomerID allows Spanner to perform an index scan followed by a point read, drastically reducing latency for these queries.
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 CustomerID.
Why this is correct
A secondary index allows direct lookup by CustomerID, significantly reducing read 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 interleaved tables.
Why it's wrong here
Interleaving is for storing child rows with parents; it does not improve point read performance for non-key columns.
✗
Reorder primary key to (OrderDate, CustomerID).
Why it's wrong here
Reordering does not optimize for CustomerID-only lookups because the index is not used efficiently.
✗
Increase the number of nodes.
Why it's wrong here
Adding nodes increases throughput but does not reduce latency for individual point reads.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume reordering the primary key (Option C) is a valid optimization, but Cloud Spanner's design requires the full primary key for efficient point reads, and changing the key order does not eliminate the need for a secondary index when filtering on a prefix alone.
Detailed technical explanation
How to think about this question
Cloud Spanner uses a distributed storage layer where primary key rows are split across splits based on the key order. Without a secondary index, a query filtering only on CustomerID triggers a full table scan across all splits. A secondary index on CustomerID creates a separate index table that maps CustomerID to the base table's primary key, enabling Spanner to locate rows via a single split lookup and then fetch the full row with a point read.
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.
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: Add a secondary index on CustomerID. — Point reads by CustomerID only are inefficient on the primary key (CustomerID, OrderDate) because Cloud Spanner requires the full primary key for direct lookup. Adding a secondary index on CustomerID allows Spanner to perform an index scan followed by a point read, drastically reducing latency for these queries.
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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