The answer is that the table is not partitioned, or the partitions are not properly defined in the table DDL. This is the most likely cause of slow queries because Athena relies on partition pruning to skip irrelevant data—if the table schema does not correctly map the S3 folder structure (year, month, day, hour) as partition columns, Athena scans the full 10 TB instead of only the necessary 2 TB. On the AWS Certified Data Engineer Associate DEA-C01 exam, this question tests your understanding of how Athena’s partition pruning works and how misconfigured table definitions lead to excessive data scanning, a common trap where candidates blame file format or compression instead. Remember the key insight: partition pruning only functions when the DDL explicitly defines the partition columns and the data is physically organized in the corresponding S3 prefixes. A useful memory tip is “DDL defines the cut”—if your table DDL doesn’t declare the partition keys, Athena will scan the whole dataset, no matter how well your S3 folders are organized.
DEA-C01 Data Operations and Support Practice Question
This DEA-C01 practice question tests your understanding of data operations and support. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 company uses Amazon S3 to store log files from multiple sources. The logs are partitioned by year, month, day, and hour. A data engineer uses Amazon Athena to query the logs. Recently, users have reported that queries are taking longer than expected. The engineer notices that many queries are scanning large amounts of data even when filtering on partition columns. The total data size is 10 TB, and the average query scans 2 TB. The partition columns are properly defined in the table schema. What is the most likely cause of the slow queries?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The table is not partitioned, or the partitions are not properly defined in the table DDL.
Option A is correct because Athena queries only use partition pruning if the table is partitioned by those columns and the data is organized accordingly. If the table is not partitioned, Athena scans all data, leading to high scan volumes. Option B is wrong because file compression reduces scan size, not increases it. Option C is wrong because the number of partitions does not cause high scan volume; it helps reduce it. Option D is wrong because the data format (CSV vs. Parquet) affects performance but not partition pruning.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 number of partitions is too large, causing Athena to spend time listing partitions.
Why it's wrong here
A large number of partitions can cause overhead but does not increase the amount of data scanned.
✓
The table is not partitioned, or the partitions are not properly defined in the table DDL.
Why this is correct
Without proper partitions, Athena scans the entire dataset, causing high scan volumes and slow queries.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The log files are stored in compressed format (e.g., gzip), which increases the amount of data scanned.
Why it's wrong here
Compression reduces the amount of data scanned by Athena.
✗
The log files are stored in CSV format instead of columnar formats like Parquet.
Why it's wrong here
While columnar formats are more efficient, the primary issue is partition pruning.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Identify which DEA-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Data Operations and Support — This question tests Data Operations and Support — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The table is not partitioned, or the partitions are not properly defined in the table DDL. — Option A is correct because Athena queries only use partition pruning if the table is partitioned by those columns and the data is organized accordingly. If the table is not partitioned, Athena scans all data, leading to high scan volumes. Option B is wrong because file compression reduces scan size, not increases it. Option C is wrong because the number of partitions does not cause high scan volume; it helps reduce it. Option D is wrong because the data format (CSV vs. Parquet) affects performance but not partition pruning.
What should I do if I get this DEA-C01 question wrong?
Identify which DEA-C01 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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