Partial-match retrieval using hashing and descriptors

This paper studies a partial-match retrieval scheme based on hash functions and descriptors. The emphasis is placed on showing how the use of a descriptor file can improve the performance of the scheme. Records in the file are given addresses according to hash functions for each field in the record. Furthermore, each page of the file has associated with it a descriptor, which is a fixed-length bit string, determined by the records actually present in the page. Before a page is accessed to see if it contains records in the answer to a query, the descriptor for the page is checked. This check may show that no relevant records are on the page and, hence, that the page does not have to be accessed. The method is shown to have a very substantial performance advantage over pure hashing schemes, when some fields in the records have large key spaces. A mathematical model of the scheme, plus an algorithm for optimizing performance, is given.