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

        return new VariableWidthBlock(positionOffset, length, sliceOutput.slice(), offsets, hasNullValue ? valueIsNull : null);
    }