DP-300 Practice Question: Monitor, configure, and optimize database resources
You are monitoring an Azure SQL Database using Query Performance Insight. You see a query with high duration and high CPU usage. The query plan shows a clustered index scan. What is the most likely cause and recommendation?
⚠ Common exam trap
Candidates often confuse a clustered index scan with fragmentation or parameter sniffing, but the scan is a symptom of a missing nonclustered index that would allow a seek, not a problem with the clustered index itself or plan caching.
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
✓
Missing nonclustered index; create an index on the predicates.
Query Performance Insight shows a query with high duration and CPU usage, and the query plan reveals a clustered index scan. A clustered index scan reads all rows in the table, which is inefficient when only a subset of rows is needed. The most likely cause is a missing nonclustered index on the columns used in the WHERE clause (predicates), which would allow a seek operation instead of a full scan, reducing both CPU and duration.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Fragmented clustered index; rebuild the clustered index.
Why it's wrong here
Fragmentation causes scans but not necessarily high CPU; missing index is more likely.
- ✗
Insufficient memory; increase the service tier.
Why it's wrong here
Memory pressure might cause scans, but missing index is a common cause.
- ✓
Missing nonclustered index; create an index on the predicates.
Why this is correct
An index seek would reduce CPU and duration.
- ✗
Parameter sniffing; add OPTION (RECOMPILE).
Why it's wrong here
Parameter sniffing causes varying plans, not necessarily scans.
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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