**Why transformations matter**: Choice affects Catalyst optimization and performance. **Common**: map, filter, select, withColumn, drop, groupBy, agg, join, union, distinct. Window: row_number, rank, sum over. **Scalability trade-offs**: Narrow (map, filter) = no shuffle; wide (groupBy, join) = shuffle. Prefer narrow; minimize wide. **Cost implications**: UDF breaks optimization; built-ins preferred....
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