Criss-Cross Hash Joins: Design and Analysis

Join processing in relational database systems continues to be a difficult and challenging problem. In this research, we propose a criss-cross hash join strategy that draws from both hashing and indexing techniques, inheriting the advantages of each. To facilitate the criss-cross hash join, a simple data structure, termed page map, is introduced. The page maps aid in reducing the hashing effort incurred in the current hash based join methods. Furthermore, the page maps implicitly capture and exploit the possible inherent order among tuples in the relations, however partial it may be, to achieve superior performance. As the proposed methodology relies on the hashing scheme, the page maps are simpler, more compact, and easier to maintain than the traditional data structures associated with index based join methods. We develop the ideas intuitively first, followed by a formal development of the concepts and the algorithms. A detailed probabilistic analysis of the algorithms is presented and their performance is assessed through extensive empirical investigations. The empirical analysis suggests significant performance improvements over the current state-of-the-art hybrid hash method, especially in the presence of possible inherent order.