MLS-C01 Exploratory Data Analysis Practice Question
Network Topology
Refer to the exhibit. A data scientist queries the table with 'SELECT COUNT(*) FROM mytable' in Athena and gets a result of 1000 rows. However, the scientist knows there are 1500 data files in the S3 location. What is the most likely reason for the discrepancy?
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
✓
Some files may be empty or contain only headers, so they contribute 0 rows.
Athena counts rows from data files; if files are empty or contain only headers, they contribute 0 rows. With 1500 files and only 1000 rows, it is plausible that many files are empty or header-only, especially if the data pipeline produces such files. Option A is incorrect because Athena still parses lines even with a delimiter mismatch, treating each line as a row (though columns may be incorrect). Option B is incorrect because schema mismatch typically causes query errors, not silent skipping. Option D is false because Athena does not skip files based on size limits.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Some files may use a different delimiter (e.g., tab) and are not parsed correctly, resulting in zero rows from those files.
Why it's wrong here
Incorrect. Even with a different delimiter, Athena would still parse each line as a row, but columns may be wrong; it does not skip rows entirely.
- ✗
The table schema does not match the data, causing some files to be skipped.
Why it's wrong here
Incorrect. Schema mismatch usually results in query failures, not silent omission of rows.
- ✓
Some files may be empty or contain only headers, so they contribute 0 rows.
Why this is correct
Correct. Files that are empty or contain only a header row contribute zero data rows, explaining the discrepancy.
- ✗
Athena skips files larger than a certain size to prevent scanning too much data.
Why it's wrong here
Incorrect. Athena scans all files regardless of size unless explicitly filtered via partitioning or WHERE clauses.
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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