MLS-C01 Exploratory Data Analysis Practice Question
Network Topology
A data scientist queried an Athena table and got only one row back, but the CSV file is 1 MB. What is the most likely reason?
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 is partitioned but the partition is not correctly defined
A 1 MB CSV file likely contains many rows, but querying returns only one row, which indicates that the table's partition mapping is incorrect. Athena uses partitions to minimize data scanned; if the partition definition does not match the actual data location, the query may only read (or miss) certain partitions, resulting in fewer rows. Option B is wrong because a 1 MB file is too large to contain only one row, as typical CSV rows are much smaller. Option C is irrelevant: whether the table is external does not affect row count. Option D is wrong because Athena supports CSV format.
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 table is partitioned but the partition is not correctly defined
Why this is correct
Correct: If date partition is not correctly mapped, the filter may return no data.
- ✗
The CSV file contains only one row
Why it's wrong here
Incorrect: 1 MB file typically contains many rows.
- ✗
The table is not an external table
Why it's wrong here
Incorrect: External tables are standard for Athena.
- ✗
Athena does not support CSV format
Why it's wrong here
Incorrect: Athena supports CSV.
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