This DEA-C01 practice question tests your understanding of data store management. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Use EXPLAIN to review the query plan.
Reveals how the query is executed, helps identify bottlenecks.
Run ANALYZE on the table.
Why wrong: Updates statistics but not the first step.
Check the concurrency scaling status.
Why wrong: Concurrency scaling is for handling many concurrent queries.
Run VACUUM on the table.
Why wrong: VACUUM reclaims space, doesn't directly improve query plans.
Answer choices
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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
Reveals how the query is executed, helps identify bottlenecks.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
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.
Common exam traps
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.
Detailed technical explanation
Amazon Redshift uses a columnar storage format with zone maps that track min/max values per block; a query that scans a large table but returns few rows often fails to leverage these zone maps due to a missing or poorly written WHERE clause. The EXPLAIN output shows whether the query uses a 'Seq Scan' (full table scan) versus a 'Filter' applied early, and can reveal if the query is performing a cross-join or missing distribution key alignment. In practice, a common scenario is a query that scans a fact table of billions of rows but filters on a non-distribution key without a sort key, causing Redshift to read all blocks instead of pruning them via zone maps.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
What to study next
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FAQ
Data Store Management — This question tests Data Store Management — Read the scenario before looking for a memorised answer..
The correct answer is: 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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