presto-orc/src/main/java/io/prestosql/orc/writer/StructColumnWriter.java/StructColumnWriter/writeColumnarRow
class StructColumnWriter:    private void writeColumnarRow(ColumnarRow columnarRow)
    {
        // record nulls
        for (int position = 0; position < columnarRow.getPositionCount(); position++) {
            boolean present = !columnarRow.isNull(position);
            presentStream.writeBoolean(present);
            if (present) {
                nonNullValueCount++;
            }
        }

        // write field values
        for (int i = 0; i < structFields.size(); i++) {
            ColumnWriter columnWriter = structFields.get(i);
            Block fieldBlock = columnarRow.getField(i);
            if (fieldBlock.getPositionCount() > 0) {
                columnWriter.writeBlock(fieldBlock);
            }
        }
    }