The answer is an index on Users(Email). This is correct because the query’s WHERE clause filters on the Email column, and without that index, the database engine must perform a full table scan to locate the row. Even if the user exists, a full scan on a large table can fail to return the row due to stale statistics or uneven data distribution, whereas an index seek on Email guarantees an efficient, direct lookup. On the Google Professional Cloud Database Engineer exam, this scenario tests your understanding of how missing indexes degrade query performance and reliability, often appearing as a trap where candidates assume a row will always be found without an index. A common memory tip: “If it’s in the WHERE, it needs an index there.”
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.
Exhibit
gcloud spanner databases execute-sql my-db --sql="SELECT * FROM Users WHERE Email = 'test@example.com'"
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Output: (empty)
A user runs the above command and expects a row to be returned because the user exists. Which index is missing?
gcloud spanner databases execute-sql my-db --sql="SELECT * FROM Users WHERE Email = 'test@example.com'"
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Output: (empty)
A
Primary key index on Users(Email)
Why wrong: Primary key is not on Email, so this is irrelevant.
B
Index on Users(Email)
An index on Email allows direct lookup by email, returning the row efficiently.
C
Composite index on Users(Email, UserId)
Why wrong: While this could work, a single-column index on Email is sufficient and simpler.
D
No index needed, query scans full table.
Why wrong: A full table scan would still return the row if it exists; the empty output suggests the query is not scanning correctly due to missing index.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Index on Users(Email)
Option B is correct because the query is filtering on the `Email` column, and without an index on `Users(Email)`, the database must perform a full table scan. Even though the user exists, the query may not return a row if the table is large and the optimizer chooses a scan that misses the row due to data distribution or lack of statistics. An index on `Email` allows an index seek, ensuring the row is found efficiently.
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.
✗
Primary key index on Users(Email)
Why it's wrong here
Primary key is not on Email, so this is irrelevant.
✓
Index on Users(Email)
Why this is correct
An index on Email allows direct lookup by email, returning the row efficiently.
Related concept
Read the scenario before looking for a memorised answer.
✗
Composite index on Users(Email, UserId)
Why it's wrong here
While this could work, a single-column index on Email is sufficient and simpler.
✗
No index needed, query scans full table.
Why it's wrong here
A full table scan would still return the row if it exists; the empty output suggests the query is not scanning correctly due to missing index.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a primary key index is always present on the lookup column, but the question tests whether you recognize that a non-primary key column needs its own index for efficient filtering, not that the primary key itself is missing.
Trap categories for this question
Command / output trap
A full table scan would still return the row if it exists; the empty output suggests the query is not scanning correctly due to missing index.
Detailed technical explanation
How to think about this question
In databases like PostgreSQL or MySQL, an index on `Email` creates a B-tree structure that allows O(log n) lookup time. Without it, the query planner must scan all rows, which can lead to missed rows if the table is partitioned or if the scan is interrupted by a lock. In real-world scenarios, missing an index on a frequently filtered column like email can cause timeouts in high-traffic applications, as seen in login systems where a user lookup must be fast and reliable.
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: Index on Users(Email) — Option B is correct because the query is filtering on the `Email` column, and without an index on `Users(Email)`, the database must perform a full table scan. Even though the user exists, the query may not return a row if the table is large and the optimizer chooses a scan that misses the row due to data distribution or lack of statistics. An index on `Email` allows an index seek, ensuring the row is found efficiently.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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