DEA-C01 Data Operations and Support Practice Question
A company uses Amazon Athena to query data stored in an S3 bucket. The data is partitioned by year, month, day, and hour. The data engineer notices that queries are scanning a large amount of data even with a WHERE clause on the partition columns. 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 partitions defined in the AWS Glue Data Catalog.
If partitions are not defined in the table, Athena cannot perform partition pruning. Option A is wrong because too many partitions improve pruning, not hinder scanning. Option C is wrong because S3 storage class does not affect scanning. Option D is wrong because compressed files reduce scan size, not increase.
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 data has too many partitions, causing overhead.
Why it's wrong here
While too many partitions can cause some overhead in partition discovery, the primary issue described is that queries scan large amounts of data despite a WHERE clause, which indicates partitions are not properly defined in the catalog.
- ✓
The table does not have partitions defined in the AWS Glue Data Catalog.
Why this is correct
If partitions are not defined in the AWS Glue Data Catalog, Athena cannot perform partition pruning, leading to full table scans.
- ✗
The S3 bucket uses the S3 Glacier storage class.
Why it's wrong here
The S3 storage class (e.g., Glacier) does not affect Athena's ability to prune partitions; it may impact data retrieval time but not the amount of data scanned.
- ✗
The data files are compressed with GZIP.
Why it's wrong here
Compressed files (e.g., GZIP) reduce the size of data scanned, not increase it.
Quick reference
AWS S3 Storage Class Comparison
| Storage Class | Min Duration | Retrieval | Use Case |
|---|---|---|---|
| S3 Standard | None | Immediate | Frequently accessed data |
| S3 Standard-IA | 30 days | Immediate | Infrequent access, rapid retrieval |
| S3 One Zone-IA | 30 days | Immediate | Non-critical infrequent data |
| S3 Intelligent-Tiering | None | Immediate–hours | Unknown or changing access patterns |
| S3 Glacier Instant | 90 days | Milliseconds | Archive with instant retrieval |
| S3 Glacier Flexible | 90 days | Minutes–hours | Archive, flexible retrieval |
| S3 Glacier Deep Archive | 180 days | Hours | Long-term compliance archive |
Go deeper
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Same concept, more angles
1 more way this is tested on DEA-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A data engineer is running an Amazon Athena query that scans a large amount of data in Amazon S3, resulting in high costs. The data is stored in Parquet format in a partitioned table. Which strategy would be MOST effective in reducing the amount of data scanned?
medium- ✓ A.Ensure the query includes a WHERE clause that filters on partition columns.
- B.Convert the Parquet files to CSV format and apply GZIP compression.
- C.Use S3 Intelligent-Tiering storage class to reduce storage costs.
- D.Increase the number of partitions by adding more partition columns.
Why A: Partition pruning allows Athena to read only the partitions that match the WHERE clause, significantly reducing the amount of data scanned. Option A is correct because filtering on partition columns is the most effective way to minimize scanned data. Option B is incorrect because Parquet is a columnar format that already compresses well and reduces scan compared to CSV with GZIP. Option C is incorrect because S3 Intelligent-Tiering optimizes storage costs, not query scan costs. Option D is incorrect because adding more partition columns does not reduce scan unless the query filters on them, and may increase metadata overhead.
JA
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.