presto-spi/src/main/java/io/prestosql/spi/block/VariableWidthBlockEncoding.java/VariableWidthBlockEncoding/readBlock
class VariableWidthBlockEncoding:    @Override
    public Block readBlock(BlockEncodingSerde blockEncodingSerde, SliceInput sliceInput)
    {
        int positionCount = sliceInput.readInt();

        int[] offsets = new int[positionCount + 1];
        sliceInput.readBytes(Slices.wrappedIntArray(offsets), SIZE_OF_INT, positionCount * SIZE_OF_INT);

        boolean[] valueIsNull = decodeNullBits(sliceInput, positionCount).orElse(null);

        int blockSize = sliceInput.readInt();
        Slice slice = sliceInput.readSlice(blockSize);

        return new VariableWidthBlock(0, positionCount, slice, offsets, valueIsNull);
    }