presto-rcfile/src/main/java/io/prestosql/rcfile/RcFileReader.java/Column/readBlock
class Column:        public Block readBlock(int position, int size)
                throws IOException
        {
            checkArgument(size > 0 && size <= ColumnData.MAX_SIZE, "Invalid size");
            checkArgument(currentPosition <= position, "Invalid position");

            if (currentPosition < position) {
                skipTo(position);
            }

            // read offsets
            int[] offsets = readOffsets(size);

            ColumnData columnData = new ColumnData(offsets, getDataBuffer());
            Block block = encoding.decodeColumn(columnData);
            return block;
        }