**Shuffle**: Redistribution for groupBy, join, distinct. Wide transformation; data moves across network. Expensive—serialization, network, disk.
**Handle**: (1) **Reduce data**—select only needed columns before shuffle. (2) **Broadcast** small tables. (3) **Partition by key** when writing—next read avoids shuffle. (4) **Repartition** before multiple shuffles—one repartition vs. many. (5) **Coalesce** after filter to reduce output. (6) **Salting** for skew....
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