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

        return new VariableWidthBlock(positionOffset + arrayOffset, length, slice, offsets, valueIsNull);
    }