**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Delta large-scale updates: MERGE for upserts—matches on key, updates/inserts. Partition pruning and file skipping via metrics. OPTIMIZE plus ZORDER for read performance. Example: MERGE INTO target USING source ON target.id=source.id WHEN MATCHED UPDATE SET * WHEN NOT MATCHED INSERT *....
The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations. It covers performance optimization strategies, common pitfalls to avoid, and real-world examples from companies like PWC. The answer also includes follow-up discussion points that interviewers commonly explore.
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