USING SEMI-JOINS TO REALIZE JOINS AND MIXED-SEMI(?)JOINS

In DDBS query processing, Semi-Join is often used to realize joins of relations distributed among different sites. This method may reduce communication cost in performing joins. This paper provides an efficient algorithm to perform this kind of operations. In comparison wish the existing methods, our algorithm has the following advantages. (1) An useful and practical criterion J to determine whether a Semi-Join is beneficial and to measure its benefit. (2) With the help of J, using the Semi-Join, to perform the corresponding Join or the Join dorectly before sending data. This paper generalizes Semi-Joins by putting forward the concept of Mixed-Somi-Joins(MSJs). The properties about MSJs are discussed. They are useful to prowess more general queries efficiently. Such as the selections on the Joins. All these new works provide a proper base to solve the queries in the form of the Select-Project-Join expression.