External perfect hashing

Abstract

A hashing function is perfect if it does not create any overflow records The use of perfect hashing functions has previously been studied only for small static sets stored m mam memory In this paper we describe a perfect hashing scheme for large external files which we are currently mvestigatmg The scheme guarantees retrieval of any record in a single disk access This is achieved at the cost of a small m-core table and increased cost of insertions We also suggest a pohcy for limrtmg the cost of msertrons and we study the tradeoff between expected storage utthzatron, size of the internal table and cost of msertrons under this pohcy The results obtained so far are very promrsmg They indicate that it may indeed by posstble to destgn practical perfect hashing schemes for external files based on the suggested approach. © 1985, ACM. All rights reserved.