**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Catalyst Optimizer improves performance via: (1) Logical optimization—predicate pushdown, constant folding, projection pruning. (2) Physical planning—join strategy (broadcast, sort-merge), partition pruning. (3) Code generation—whole-stage codegen for faster execution. Example: `df.filter('x>0').select('a')` pushes filter to source, prunes columns....
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 American Express. The answer also includes follow-up discussion points that interviewers commonly explore.
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