presto-spi/src/main/java/io/prestosql/spi/block/AbstractRowBlock.java/AbstractRowBlock/getRegion
class AbstractRowBlock:    @Override
    public Block getRegion(int position, int length)
    {
        int positionCount = getPositionCount();
        checkValidRegion(positionCount, position, length);

        return createRowBlockInternal(position + getOffsetBase(), length, getRowIsNull(), getFieldBlockOffsets(), getRawFieldBlocks());
    }