**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Schema evolution in PySpark: (1) `mergeSchema` in read/write. (2) `schema_of_json` for JSON. (3) `StructType` with `nullable=True`. (4) `withColumn` for new columns. (5) Validate with `df.schema`....
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 Nihilent. The answer also includes follow-up discussion points that interviewers commonly explore.
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