DEA-C01 Data Store Management Practice Question
A data engineer is troubleshooting a slow-running query on Amazon Redshift. The query scans a large table but returns few rows. Which diagnostic step should be taken first?
⚠ Common exam trap
The trap here is that candidates often jump to performance-tuning commands like ANALYZE or VACUUM without first diagnosing the query plan, but the DEA-C01 exam emphasizes that EXPLAIN is the foundational step for identifying inefficient scan patterns before applying any corrective actions.
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
✓
Use EXPLAIN to review the query plan.
When a query scans a large table but returns few rows, the most likely cause is an inefficient query plan—such as a full table scan instead of using indexes or zone maps. Using EXPLAIN first reveals the execution plan, allowing the engineer to identify whether the query is performing unnecessary sequential scans, missing filter pushdown, or using suboptimal join strategies. This diagnostic step should always precede tuning actions like ANALYZE or VACUUM, which address data distribution or storage bloat rather than query planning.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use EXPLAIN to review the query plan.
Why this is correct
EXPLAIN reveals the query plan, exposing sequential scans, missing sort keys, or distribution distortions causing the large scan. Reviewing it first identifies the actual bottleneck before altering the query or cluster, satisfying the stem's requirement to diagnose the slow query.
- ✗
Run ANALYZE on the table.
Why it's wrong here
Updates statistics but not the first step.
- ✗
Check the concurrency scaling status.
Why it's wrong here
Concurrency scaling is for handling many concurrent queries.
- ✗
Run VACUUM on the table.
Why it's wrong here
VACUUM reclaims space, doesn't directly improve query plans.
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