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Question 10 of 1,730

Management and Operations →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the query optimizer chooses a sequential scan over an index scan when it estimates the query will return a large percentage of rows from the table. This decision hinges on the cost-based optimizer’s evaluation of random I/O versus sequential I/O: reading many rows scattered across disk pages via an index is more expensive than a single sequential pass through the entire table, especially when the selectivity is low. On the AWS Certified Database Specialty DBS-C01 exam, this concept tests your understanding of how PostgreSQL’s planner weighs row estimate percentages against index overhead—a common trap is assuming an index is always faster, but the optimizer prioritizes total I/O cost. Remember the memory tip: “Sequential for the many, index for the few”—if the WHERE clause filters out most rows, an index wins; if it returns a large chunk, a sequential scan is cheaper.

DBS-C01 Management and Operations Practice Question

This DBS-C01 practice question tests your understanding of management and operations. 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.

A database administrator is troubleshooting a slow-running query on an Amazon RDS for PostgreSQL DB instance. The query plan shows a sequential scan on a large table. The table has a primary key and an index on the column used in the WHERE clause. Why might the query optimizer choose a sequential scan over an index scan?

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.

Question 1mediummultiple choice

Full question →

A
The query is expected to return a large percentage of rows.
When a query returns a significant portion of rows, a sequential scan is often faster than an index scan because it avoids many random I/O operations.
B
The query is a SELECT * without a WHERE clause.
Why wrong: The stem says the query has a WHERE clause.
C
The table's statistics are outdated.
Why wrong: Outdated statistics can cause the optimizer to make poor choices, but the question asks for a reason why it might choose sequential scan; outdated stats could lead to either plan.
D
The index on the column is not being used because it is a composite index with a different column order.
Why wrong: The stem states there is an index on the column used in the WHERE clause, so column order is not an issue.

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 is expected to return a large percentage of rows.

Option B is correct because if the optimizer estimates that a large percentage of rows will be returned, a sequential scan can be more efficient than random I/O from an index. Option A is wrong because outdated statistics can cause suboptimal plans, but the question asks for a reason the planner might choose sequential scan; outdated stats could lead to either choice. Option C is wrong because the presence of an index does not force its use; the optimizer decides. Option D is wrong because the query has a WHERE clause, so a full table scan is not the only option, but it may be chosen due to the selectivity.

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 is expected to return a large percentage of rows.
Why this is correct
When a query returns a significant portion of rows, a sequential scan is often faster than an index scan because it avoids many random I/O operations.
Clue confirmation
The clue word "primary" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The query is a SELECT * without a WHERE clause.
Why it's wrong here
The stem says the query has a WHERE clause.
✗
The table's statistics are outdated.
Why it's wrong here
Outdated statistics can cause the optimizer to make poor choices, but the question asks for a reason why it might choose sequential scan; outdated stats could lead to either plan.
✗
The index on the column is not being used because it is a composite index with a different column order.
Why it's wrong here
The stem states there is an index on the column used in the WHERE clause, so column order is not an issue.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

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.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
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 DBS-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this DBS-C01 question test?

Management and Operations — This question tests Management and Operations — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The query is expected to return a large percentage of rows. — Option B is correct because if the optimizer estimates that a large percentage of rows will be returned, a sequential scan can be more efficient than random I/O from an index. Option A is wrong because outdated statistics can cause suboptimal plans, but the question asks for a reason the planner might choose sequential scan; outdated stats could lead to either choice. Option C is wrong because the presence of an index does not force its use; the optimizer decides. Option D is wrong because the query has a WHERE clause, so a full table scan is not the only option, but it may be chosen due to the selectivity.

What should I do if I get this DBS-C01 question wrong?

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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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.