presto-main/src/main/java/io/prestosql/operator/PartitionedOutputOperator.java/PagePartitioner/getSizeInBytes
class PagePartitioner:        public long getSizeInBytes()
        {
            // We use a foreach loop instead of streams
            // as it has much better performance.
            long sizeInBytes = 0;
            for (PageBuilder pageBuilder : pageBuilders) {
                sizeInBytes += pageBuilder.getSizeInBytes();
            }
            return sizeInBytes;
        }