ABSTRACT. The semi-join is a relational algebraic operation that selects a set of tuples in one relation that match one or more tuples of another relation on the joining domains. Semi-joins have been used as a basic ingredient in query processing strategies for a number of hardware and software database systems. However, not all queries can be solved entirely using semi-joins. In this paper the exact class of relational queries that can be solved using semi-joins is shown. It is also shown that queries outside of this class may not even be partially solvable using "short " semi-join programs. In addition, a linear-time membership test for this class is presented.

This paper presents a new data structure for multiway and general join query acceleration, the hit-list, and an algorithm for its use. The hit-list is a surrogate index providing the mapping between the values of two attributes in a relation participating in an equijoin or a selection. The results of an analytical model, simulation study, and an implementation are presented. The performance advantages of this approach are made clear, as well as the basis for these results in the attainment of full selectivity. Extensions of hit-lists are also examined. 1. Introduction This paper presents a new data structure, the hit-list, which can be used to accelerate complex queries. The hit-list acceleration method is developed and applied to multi-way joins in which not all of the joining attributes are the same. An analytical model was developed which compared the response time of a multi-way join algorithm using hit-lists with the performance of an algorithm not using hit-lists. The comparison ...