Function aggregate_scan_metrics 

pub fn aggregate_scan_metrics(plan: &dyn ExecutionPlan) -> ScanMetrics

Walk the physical-plan tree: output_rows from the root node (final result grain), bytes_scanned/spilled_bytes/spill_count summed across every node (leaf DataSourceExec nodes carry bytes_scanned; spilling operators such as ExternalSorter carry the other two). sum_by_name cannot be used for the spill counters: MetricsSet::sum_by_name explicitly returns false for MetricValue::SpillCount/SpilledBytes, so it would silently report zero; the dedicated MetricsSet::spill_count()/spilled_bytes() accessors are used instead.