presto-main/src/main/java/io/prestosql/operator/PartitionedOutputOperator.java/PartitionedOutputOperator/toString
class PartitionedOutputOperator:        @Override
        public String toString()
        {
            return toStringHelper(this)
                    .add("rowsAdded", rowsAdded)
                    .add("pagesAdded", pagesAdded)
                    .add("outputBufferPeakMemoryUsage", outputBufferPeakMemoryUsage)
                    .toString();
        }