DP-300 Plan and implement data platform resources Practice Question
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?
⚠ Common exam trap
Many candidates 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.
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.
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.
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.
- ✗
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.
Go deeper
Related to this question
Learn chapter
Overview of Azure Data Platform Options
Key term
Azure SQL Indexes
Structures in Azure SQL Database that speed up data retrieval by providing quick access paths to rows, similar to a book index.
Key term
Azure SQL Performance Tuning
Azure SQL Performance Tuning is the process of optimizing the speed and efficiency of queries and database operations in Microsoft Azure SQL Database or SQL Managed Instance to reduce latency and improve throughput.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DP-300 practice question is part of Courseiva's free Microsoft 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 DP-300 exam.