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Workload-Specific Database Design →mediumMultiple ChoiceObjective-mapped

DBS-C01 Workload-Specific Database Design Practice Question

This DBS-C01 practice question tests your understanding of workload-specific database design. 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 company uses Amazon Aurora MySQL for its customer relationship management (CRM) system. The database has a table "contacts" with millions of rows. The application frequently searches for contacts by email address. The email column has a B-tree index. The DBA notices that queries are still slow, and the EXPLAIN plan shows index scans but not index-only scans. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The query selects columns not included in the index, requiring table lookups.
If the query selects columns like phone not in the index, the database must access the table, preventing an index-only scan.
B
The index is a composite index on (email, phone) and the query selects only email.
Why wrong: A composite index on (email, phone) would still be used for email-only queries, but it may be larger.
C
The index has low cardinality.
Why wrong: Low cardinality would cause many rows per key, but that leads to index scans, not index-only scans.
D
The index type is not suitable for equality searches.
Why wrong: B-tree indexes are excellent for equality searches.

Full breakdown with real-world context →

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

✓

The query selects columns not included in the index, requiring table lookups.

The correct answer is A because an index-only scan requires that all columns referenced in the query (both in the SELECT list and WHERE clause) be present in the index. Since the query selects columns not included in the B-tree index on the email column, Aurora MySQL must perform additional table lookups (row fetches) to retrieve those missing columns, resulting in an index scan rather than an index-only scan.

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.

✓
The query selects columns not included in the index, requiring table lookups.
Why this is correct
If the query selects columns like phone not in the index, the database must access the table, preventing an index-only scan.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The index is a composite index on (email, phone) and the query selects only email.
Why it's wrong here
A composite index on (email, phone) would still be used for email-only queries, but it may be larger.
✗
The index has low cardinality.
Why it's wrong here
Low cardinality would cause many rows per key, but that leads to index scans, not index-only scans.
✗
The index type is not suitable for equality searches.
Why it's wrong here
B-tree indexes are excellent for equality searches.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume any index scan is optimal, failing to recognize that an index-only scan (covered index) is significantly faster because it avoids table row lookups, and the EXPLAIN plan's 'Using index' vs. 'Using index condition' distinction is the key clue.

Detailed technical explanation

How to think about this question

In InnoDB (used by Aurora MySQL), secondary indexes store the primary key value as a pointer to the row. An index-only scan avoids accessing the clustered index (table) only if the query's columns are fully covered by the secondary index. When a query selects columns not in the index, MySQL must perform a 'double lookup'—first the secondary index, then the clustered index—which adds random I/O and slows performance. This is a common scenario where adding a covering index (e.g., on (email, selected_columns)) can dramatically improve query speed.

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.

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FAQ

Questions learners often ask

What does this DBS-C01 question test?

Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The query selects columns not included in the index, requiring table lookups. — The correct answer is A because an index-only scan requires that all columns referenced in the query (both in the SELECT list and WHERE clause) be present in the index. Since the query selects columns not included in the B-tree index on the email column, Aurora MySQL must perform additional table lookups (row fetches) to retrieve those missing columns, resulting in an index scan rather than an index-only scan.

What should I do if I get this DBS-C01 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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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This DBS-C01 practice question is part of Courseiva's free Amazon Web Services 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 DBS-C01 exam.