Exploiting clustering in inverted file compression

Document databases contain large volumes of text, and currently have typical sizes into the gigabyte range. In order to efficiently query these text collections some form of index is required, since without an index even the fastest of pattern matching techniques results in unacceptable response times. One pervasive indexing method is the use of inverted files, also sometimes known as concordances or postings files. There has been a number of effort made to capture the "clustering" effect, and to design index compression methods that condition their probability predictions according to context. In these methods information as to whether or not the most recent (or second most recent, and so on) document contained term t is used to bias the prediction that the next document will contain term t. We further extend this notion of context-based index compression, and describe a surprisingly simple index representation that gives excellent performance on all of our test databases; allows fast decoding; and is, even in the worst case, only slightly inferior to Golomb (1966) coding.