Question 286 of 963

Plan and implement data platform resources →hardMultiple ChoiceObjective-mapped

DP-300 Plan and implement data platform resources Practice Question

This DP-300 practice question tests your understanding of plan and implement data platform resources. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.

You have an Azure SQL Database with a table that stores historical data. To improve query performance, you create a nonclustered columnstore index on the table. However, after a large data load, you notice that query performance is worse than before. What is the most likely reason?

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 columnstore index is fragmented and needs to be rebuilt.
Why wrong: Fragmentation is less of an issue for columnstore; the main issue is rowgroup size.
B
The data was loaded in small batches, causing rowgroups to be smaller than optimal.
Small rowgroups reduce compression and query performance.
C
The table has too many columns included in the index.
Why wrong: Columnstore can include many columns; this is not the likely cause.
D
The index was created with compression disabled.
Why wrong: Columnstore indexes are always compressed.

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 data was loaded in small batches, causing rowgroups to be smaller than optimal.

Option B is correct because when data is loaded in small batches into a columnstore index, each batch may form its own rowgroup. Rowgroups with fewer than 102,400 rows are considered 'delta rowgroups' and are not compressed into the columnstore format, leading to poor compression and suboptimal query performance. The large data load likely consisted of many small batches, resulting in numerous small rowgroups that degrade scan performance.

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 columnstore index is fragmented and needs to be rebuilt.
Why it's wrong here
Fragmentation is less of an issue for columnstore; the main issue is rowgroup size.
✓
The data was loaded in small batches, causing rowgroups to be smaller than optimal.
Why this is correct
Small rowgroups reduce compression and query performance.
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 table has too many columns included in the index.
Why it's wrong here
Columnstore can include many columns; this is not the likely cause.
✗
The index was created with compression disabled.
Why it's wrong here
Columnstore indexes are always compressed.

Common exam traps

Common exam trap: answer the scenario, not the keyword

The trap here is that candidates often assume any performance degradation after an index creation is due to fragmentation (Option A), but the DP-300 exam specifically tests the understanding that columnstore indexes require large batch loads to achieve optimal rowgroup size and compression.

Detailed technical explanation

How to think about this question

Columnstore indexes store data in column segments within rowgroups, each ideally containing 102,400 to 1,048,576 rows. When rowgroups are smaller than 102,400 rows, they remain in delta stores as uncompressed B-tree structures, bypassing the columnstore's batch mode processing and vectorized operations. This forces queries to fall back to row-mode execution, which is significantly slower for analytical workloads. In real-world scenarios, ETL processes that insert rows one at a time or in small batches can inadvertently create hundreds of delta rowgroups, crippling performance until a REORGANIZE or REBUILD merges them into compressed rowgroups.

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 DP-300 question test?

Plan and implement data platform resources — This question tests Plan and implement data platform resources — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The data was loaded in small batches, causing rowgroups to be smaller than optimal. — Option B is correct because when data is loaded in small batches into a columnstore index, each batch may form its own rowgroup. Rowgroups with fewer than 102,400 rows are considered 'delta rowgroups' and are not compressed into the columnstore format, leading to poor compression and suboptimal query performance. The large data load likely consisted of many small batches, resulting in numerous small rowgroups that degrade scan performance.

What should I do if I get this DP-300 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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