**Why join strategy matters**: Shuffle is the most expensive operation; choosing wrong strategy 10x+ runtime. **Broadcast join**: Small table sent to all executors; no shuffle for large table. Use when one side < spark.sql.autoBroadcastJoinThreshold (~10MB default). **Shuffle sort-merge**: Both sides shuffled by key, sorted, merged. Default for large joins. **Shuffle hash**: One side hashed in memory; other streamed—faster when build side fits....
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