Retrieval using combinations of secondary keys
With secondary keys, we can now search for things like all the recordings of “Beethoven’s work” or all the recordings titled “Violin Concerto”.
More importantly, we can use combinations of secondary keys. (e.g., find all recordings of Beethoven’s Symphony no. 9).
Without the use of secondary indexes, this request requires a very expensive sequential search through the entire file. Using secondary indexes, responding to this query is simple and quick.