code: @Override
  	public void combine(Aggregator<T, NumericColumnSummary<T>> otherSameType) {
  		NumericSummaryAggregator<T> other = (NumericSummaryAggregator<T>) otherSameType;
  
  		nullCount += other.nullCount;
  		nanCount += other.nanCount;
  		infinityCount += other.infinityCount;
  
  		if (nonMissingCount == 0) {
  			nonMissingCount = other.nonMissingCount;
  
  			min = other.min;
  			max = other.max;
  
  			sum = other.sum;
  			mean = other.mean;
  			m2 = other.m2;
  		}
  		else if (other.nonMissingCount != 0) {
  			long combinedCount = nonMissingCount + other.nonMissingCount;
  
  			min.combine(other.min);
  			max.combine(other.max);
  
  			sum.combine(other.sum);
  
  			double deltaMean = other.mean.value() - mean.value();
  			mean = mean.add(deltaMean * other.nonMissingCount / combinedCount);
  			m2 = m2.add(other.m2).add(deltaMean * deltaMean * nonMissingCount * other.nonMissingCount / combinedCount);
  
  			nonMissingCount = combinedCount;
  		}
  	}
docs: combine two aggregations.