presto-orc/src/main/java/io/prestosql/orc/reader/StructColumnReader.java/StructColumnReader/getBlocksForType
class StructColumnReader:    private Block[] getBlocksForType(int positionCount)
    {
        Block[] blocks = new Block[fieldNames.size()];

        for (int i = 0; i < fieldNames.size(); i++) {
            String fieldName = fieldNames.get(i);

            ColumnReader columnReader = structFields.get(fieldName);
            if (columnReader != null) {
                columnReader.prepareNextRead(positionCount);
                blocks[i] = blockFactory.createBlock(positionCount, columnReader::readBlock, true);
            }
            else {
                blocks[i] = RunLengthEncodedBlock.create(type.getFields().get(i).getType(), null, positionCount);
            }
        }
        return blocks;
    }