Record Addition
When we add a record, both the data file and the index should be updated.
In the data file, the record can be added anywhere. However, the byte-offset of the new record should be saved.
Since the index is sorted, the location of the new record does matter: we have to shift all the records that belong after the one we are inserting to open up space for the new record. However, this operation is not too costly as it is performed in memory.