presto-main/src/main/java/io/prestosql/operator/PartitionedOutputOperator.java/PagePartitioner/getRetainedSizeInBytes
class PagePartitioner:        public long getRetainedSizeInBytes()
        {
            long sizeInBytes = 0;
            for (PageBuilder pageBuilder : pageBuilders) {
                sizeInBytes += pageBuilder.getRetainedSizeInBytes();
            }
            return sizeInBytes;
        }