**Shuffle**: Redistribution for join, groupBy, distinct. Wide transformation; network and serialization cost.
**Minimize**: (1) Broadcast small tables. (2) Column pruning before shuffle. (3) Filter early. (4) Partition by key when writing. (5) Repartition by key before multiple shuffles. (6) Salting for skew. (7) AQE coalesce. (8) Increase shuffle partitions if partitions too large.
**Why It Matters**: Shuffle often 70–90% of job time....
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