Reverse index
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The term reverse index has more than one meaning.
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[edit] Search Engines
When a search engine tabulates all documents that contain a given word, that is called a reverse index. This is in contrast to a regular index, which contains the locations of all words within a document.[1][2]
[edit] Key Reversal
This is a database index that uses the reversal of the key values rather than the values themselves.[citations needed]
This is particularly of value when a column in a database relation contains text values that commonly start with some fairly long prefix.
For instance, an indexed column may have a standardized form; sales orders might be formatted as a 12 character string where the first order is "SO-100000000", the second "SO-100000001", and so forth. Until there are millions of orders, the first six characters will be redundant, and won't contribute at all to index searches.
A reverse index would store these in reverse order, thus "000000001-OS", "100000001-OS", "200000001-OS", and so forth, allowing the highly-variable last character in the column to scatter the values.
[edit] Library Science
In library science, a reverse index is one where the key and result is reversed. So for the Dewey decimal system as an example, the list of decimal values and their corresponding topics is a regular index whereas a list of categories and their respective Dewey values is a reverse index.
