presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/storage/OrcStorageManager.java/OrcStorageManager/createWriterIfNecessary
class OrcStorageManager:        private void createWriterIfNecessary()
        {
            if (writer == null) {
                shardUuid = UUID.randomUUID();
                File stagingFile = storageService.getStagingFile(shardUuid);
                storageService.createParents(stagingFile);
                stagingFiles.add(stagingFile);
                writer = new OrcFileWriter(columnIds, columnTypes, stagingFile);
            }
        }