**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Columnar formats (Parquet, ORC): store by column not row. Benefits: (1) Compression—similar data compresses well; (2) Predicate pushdown—read only needed columns; (3) Better for analytics—aggregations, scans; (4) Schema embedded. Parquet is widely supported; ORC has better compression in Hive. Example: SELECT date, sum(amount) reads only those columns....
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