presto-hive/src/main/java/io/prestosql/plugin/hive/parquet/HdfsParquetDataSource.java/HdfsParquetDataSource/free
class HdfsParquetDataSource:        @Override
        public void free()
        {
            checkState(referenceCount > 0, "Reference count is already 0");

            referenceCount--;
            if (referenceCount == 0) {
                data = null;
            }
        }