DEA-C01 Data Operations and Support Practice Question
A data engineer notices that an Amazon Athena query on a partitioned table in S3 scans more data than expected. The table is partitioned by year, month, day. The query includes a WHERE clause on a non-partition column but also filters on day='2023-01-01'. What is the most likely cause of the excessive data scan?
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 partition column data type in the table definition does not match the actual partition folder names
The most likely cause is that the partition column data type in the table definition does not match the actual partition folder names. Athena uses the folder names to determine which partitions to scan (partition pruning). If the data type mismatch causes Athena to be unable to correctly interpret the folder names, partition pruning fails, and Athena scans all partitions, leading to excessive data scan. Option A is incorrect because JSON format does not prevent partition pruning; it may affect compression but not pruning. Option B is incorrect because the WHERE clause filters on a non-partition column, but it also filters on the partition column 'day', so partition pruning should work if the column data type matches. Option D is incorrect because sorting within partitions does not affect scan size; it affects query performance but not the amount of data scanned.
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 is stored in JSON format instead of Parquet
Why it's wrong here
Format affects scan but not partition pruning.
- ✗
The table is not partitioned by the column used in the WHERE clause
Why it's wrong here
It is partitioned by day, but the issue is type mismatch.
- ✓
The partition column data type in the table definition does not match the actual partition folder names
Why this is correct
If the partition column is defined as string but folders are dates, pruning fails and full scan occurs.
- ✗
The data is not sorted within partitions
Why it's wrong here
Sorting does not affect partition pruning.
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 |
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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