Why You Need an Index on Email for Efficient Lookups
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. A key principle to apply: full Table Scan. 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'"
---
Output: (empty)
A user runs the above command and expects a row to be returned because the user exists. Which index is missing?
Exhibit
gcloud spanner databases execute-sql my-db --sql="SELECT * FROM Users WHERE Email = 'test@example.com'"
---
Output: (empty)
A
Primary key index on Users(Email)
Why wrong: A primary key index on Users(Email) would be useful but is not required; the query would still find the row via a full table scan.
B
Index on Users(Email)
Why wrong: An index on Users(Email) would speed up the query but its absence does not prevent the row from being returned; a full table scan will find it.
C
Composite index on Users(Email, UserId)
Why wrong: A composite index on Users(Email, UserId) is overkill for a single equality condition; not needed.
D
No index needed, query scans full table.
No index is needed because a full table scan will correctly return the existing row. This is the correct answer.
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.”
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
✓
No index needed, query scans full table.
Option D is correct because if the user exists, a full table scan will find the row. An index on Email would improve performance but is not required for correctness. The query will return the row regardless of indexes, as long as the row exists in the table. Options A and B incorrectly suggest that a missing index prevents the row from being returned, which is not true. Option C is unnecessary because a composite index is not needed for this simple equality filter.
Key principle: Full Table Scan
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
A primary key index on Users(Email) would be useful but is not required; the query would still find the row via a full table scan.
✗
Index on Users(Email)
Why it's wrong here
An index on Users(Email) would speed up the query but its absence does not prevent the row from being returned; a full table scan will find it.
✗
Composite index on Users(Email, UserId)
Why it's wrong here
A composite index on Users(Email, UserId) is overkill for a single equality condition; not needed.
✓
No index needed, query scans full table.
Why this is correct
No index is needed because a full table scan will correctly return the existing row. This is the correct answer.
Related concept
Full Table Scan
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap is that candidates think a missing index on the filtered column will cause the row not to be returned, but a full table scan will still find the row. The question tests whether you understand that indexes affect performance, not correctness.
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
Full Table Scan
Index
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
Full Table Scan
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. Full Table Scan 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 full Table Scan, then practise related PCDE questions on the same topic to reinforce the concept.
Monitor and optimize database performance — This question tests Monitor and optimize database performance — Full Table Scan.
What is the correct answer to this question?
The correct answer is: No index needed, query scans full table. — Option D is correct because if the user exists, a full table scan will find the row. An index on Email would improve performance but is not required for correctness. The query will return the row regardless of indexes, as long as the row exists in the table. Options A and B incorrectly suggest that a missing index prevents the row from being returned, which is not true. Option C is unnecessary because a composite index is not needed for this simple equality filter.
What should I do if I get this PCDE question wrong?
Review full Table Scan, then practise related PCDE questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Full Table Scan
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.