Comprehensive and Detailed Explanation From Exact Extract of Databricks Data Engineer Documents:
Databricks documents that shuffle occurs when Spark redistributes data across partitions for grouping or joining. To optimize aggregation performance, repartitioning by the grouping key (region) ensures rows with the same key are co-located in the same partition, thus minimizing shuffle movement. Caching improves reuse of DataFrames but does not reduce shuffle volume. coalesce() reduces the number of partitions after computation and cannot prevent shuffle. Broadcast joins are unrelated to single-table aggregations. The recommended practice for reducing shuffle in aggregation is explicit repartitioning by the grouping column.