DEA-C01 Data Operations and Support Practice Question
A data engineer is troubleshooting a slow Amazon Redshift query. The EXPLAIN plan shows a 'Seq Scan' on a large table. What is the most likely cause?
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 table does not have a proper sort key defined.
A 'Seq Scan' in the EXPLAIN plan indicates a full table scan, which typically occurs when the table does not have a proper sort key defined. Without a sort key, Redshift cannot use zone maps to skip blocks, resulting in a sequential scan. Option A (too many nodes) would not cause a Seq Scan; it might improve performance if properly distributed. Option B (concurrent queries) can cause slowdown but not specifically a Seq Scan. Option D (WLM queue misconfiguration) affects query queuing and concurrency, not the scan type.
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 cluster has too many nodes.
Why it's wrong here
More nodes generally improve performance; they do not cause sequential scans.
- ✗
There are too many concurrent queries.
Why it's wrong here
Concurrency affects queue time, not the scan method chosen by the query planner.
- ✓
The table does not have a proper sort key defined.
Why this is correct
Without a sort key, Redshift performs a full table scan (Seq Scan) instead of a range-restricted scan.
- ✗
The workload management (WLM) queue is misconfigured.
Why it's wrong here
WLM controls concurrency and memory, not the scan type.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DEA-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 DEA-C01 exam.