MLS-C01 Exploratory Data Analysis Practice Question
A data scientist is performing EDA on a dataset stored in Amazon S3 using Amazon Athena. The dataset is partitioned by date, and each partition contains CSV files. The data scientist notices that some queries return zero rows for partitions that should have data. Which THREE steps should the data scientist take to troubleshoot? (Choose 3.)
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
✓
Verify that the CSV files exist in the S3 bucket for the specific partition.
Verifying that the CSV files actually exist in the S3 bucket for the specific partition confirms whether data is present. Option B is correct because MSCK REPAIR TABLE adds new partitions to the Glue Data Catalog that may not have been registered automatically. Option D is correct because incorrect data types in the query's WHERE clause can cause filters to exclude rows, resulting in zero returned rows. Option C is incorrect because converting to Parquet is not a troubleshooting step for this issue—it is an optimization. Option E is incorrect because adding a LIMIT clause does not force partition discovery; it only limits the number of rows returned.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Verify that the CSV files exist in the S3 bucket for the specific partition.
Why this is correct
Files may have been moved or deleted.
- ✓
Run MSCK REPAIR TABLE to add new partitions to the Glue Data Catalog.
Why this is correct
Partitions may not have been discovered by the crawler.
- ✗
Convert the CSV files to Parquet format.
Why it's wrong here
Format conversion is not a troubleshooting step for missing data.
- ✓
Check the data types of the columns used in the query's WHERE clause.
Why this is correct
Type mismatches can cause the filter to exclude all rows.
- ✗
Re-run the query with a LIMIT clause to force partition discovery.
Why it's wrong here
LIMIT does not trigger partition discovery.
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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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This MLS-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 MLS-C01 exam.