A lot of nonfiction books have both a Table of Contents and an Index. The Table of Contents is at the front of the book and is a roadmap through the contents, while the Index is an alphabetical list of subjects (people, places, events, and whatever else the author thinks will be of interest) along with page references to those indexed things.
But you already know that. Most of you are authors, that’s your business. So let’s talk about another kind of index.
In these days of relational databases (like the ones that run behind the scenes for almost everyone’s blog), where pretty much every column in a table is searchable, it seems archaic the way we used to handle different ways of searching through a file. These are methods that still exist, believe it or not.
Say I have an invoice file. The primary key (or index) is the invoice number, meaning if I want to look up the details of a specific invoice I need to know its number. If I don’t know the invoice number, I need to inquire against the entire file and choose the right one. Depending on the size of the file, that could take a long time.
It would help tremendously if I knew the vendor number, which just happens to be carried on the invoice record. Then, all I’d have to do would be to inquire by vendor number and list the invoices for that specific one. What I can do is to define an alternate index on the invoice file that looks at the vendor number and groups all the invoices that way. Then, instead of reading the invoice file sequentially, I can read the alternate index for that vendor and pull the records out directly.
As far as the technology we had in the 1980’s and 1990’s, that was as good as it got. Alternate indices (the plural of index) unfortunately had this tendency to get out of sync and had to be rebuilt every night just to make sure that all the records from the base file were indexed correctly. Even then, there were times when it was a shot in the dark.
If you’re really interested, here is a reaonably good tutorial on alternate indices. I only link to it because I’m pretty sure you’re not.