DEA-C01 Data Operations and Support Practice Question
A data engineer is troubleshooting a slow Amazon Redshift query. The query scans a large table with interleaved sort keys. The engineer notices that the query plan shows a sequential scan instead of a range-restricted scan. What is the MOST likely reason?
⚠ Common exam trap
The trap is blaming distribution or compression for scan-type problems; the exam tests whether you know that interleaved sort keys specifically require VACUUM REINDEX to keep zone maps usable.
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 has not been vacuumed and reindexed after large data loads.
Interleaved sort keys in Amazon Redshift rely on zone maps that must be refreshed via VACUUM REINDEX after significant data changes. Without reindexing, the zone maps become stale and the query planner cannot skip blocks, forcing a sequential scan instead of a range-restricted scan. Running VACUUM REINDEX rebuilds the interleaved sort metadata so range predicates can prune blocks effectively.
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 table has not been vacuumed and reindexed after large data loads.
Why this is correct
Interleaved sort keys rely on accurate zone maps; after large loads, unsorted or deleted rows degrade them, so Redshift cannot skip blocks and falls back to a sequential scan. Vacuum and reindex restore the range-restricted scan.
- ✗
The table has a poor distribution key (DISTKEY) causing data skew.
Why it's wrong here
A poor DISTKEY causes data skew and uneven slice workloads, not the loss of zone-map pruning that produces a sequential scan. It is tempting because distribution keys are a common Redshift tuning target, but the correct answer concerns the predicate not matching the interleaved sort key, which is what enables range-restricted scans.
- ✗
The table uses compression encodings that prevent range-restricted scans.
Why it's wrong here
Compression encodings affect storage and I/O volume, not whether Redshift can prune blocks using sort key metadata; range-restricted scans depend on the sort key and predicate alignment. It is tempting because encoding choices do influence scan performance, but the sequential scan here stems from the query not filtering on the interleaved sort key's leading column.
- ✗
The workload management (WLM) queue is configured with too few query slots.
Why it's wrong here
WLM query slots govern concurrency and queueing, not whether a scan is range-restricted; too few slots causes queries to wait, not to scan sequentially. It is tempting because WLM misconfiguration genuinely slows Redshift workloads, but the correct answer concerns the query predicate failing to match the interleaved sort key prefix.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Amazon Web Services exam blueprint
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.