Hashing schemes are widely used to improve the performance of data mining association rules, as in the DHP algorithm that utilizes the hash table in identifying the validity of candidate itemsets according to the number of the table's bucket accesses. However, since the hash table used in DHP is plagued by the collision problem, the process of generating large itemsets at each level requires two database scans, which leads to poor performance. In this paper we propose perfect hashing schemes to avoid collisions in the hash table. The main idea is to employ a refined encoding scheme, which transforms large itemsets into large 2-itemsets and thereby makes the application of perfect hashing feasible. Our experimental results demonstrate that the new method is also efficient (about three times faster than DHP), and scalable when the database size increases. We also propose another variant of the perfect hash scheme with reduced memory requirements. The properties and performances of several perfect hashing schemes are also investigated and compared.
