Indexing to provide access by multiple keys
So far, our index only allows key access. i.e., you can retrieve record DG188807, but you cannot retrieve a recording of Beethoven’s Symphony no. 9. ==> Not that useful!
We need to use secondary key fields consisting of album titles, composers, and artists.
Although it would be possible to relate a secondary key to an actual byte offset, this is usually not done (see why later). Instead, we relate the secondary key to a primary key which then will point to the actual byte offset.