**Why it matters**: At scale, design choices directly impact reliability, latency, and cost. Wrong decisions compound across jobs and teams.
Stage count = number of shuffle boundaries + 1. Shuffle (e.g., groupBy, join, distinct) creates a stage boundary. Example: `df.join(df2).groupBy('x').count()`—join causes shuffle (stage 1→2), groupBy causes shuffle (stage 2→3). So 3 stages. Stages with no shuffle (e.g., filter, map) merge with prior stage....
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