Equivalence of relational expression using generalized tableau

摘要

Query optimization is an important issue in the theory of relational databases. In query optimization, one is interested in an equivalent query but with fewer operations. Query optimization involving the minimization of logical and relational operations on different relations.rnAho, Sagiv and Ullman [ASU 79] investigated the problem for a class of relational expressions, called 'SPJ expressions'. SPJ expressions use only the relational operators select, project and join. A fairly large class of queries can be expressed using these operators. ASU introduced the notion of "tableau" to which every SPJ expression can be transformed. A tableau can be considered as tabular representation of a conjunctive query. Equivalence of SPJ - expression can be reduced to the analogous problem for tableaux. One advantage of the notation of tableaux is that there is a simple syntactic test based on the notation of 'containment mapping' and this test can be used to reduce the number of rows of tableau ASU have given a method, where for every SPJ expression with select has only equality operation and there is an equivalent tableaux. In this paper "Equivalence of relational expression using generalized tableau", We generalize the concept of the tableau to cover the condition in which general comparison operator is used. In our approach, each tableau has an allocated range table. Range table gives the range of values of each variable used in the tableaux. We focus here on the change in the concept of containment mapping. Tableau based the above generalised rules makes aviod the actual execution of the query. This aspect is especially useful in query processing in distrubuted Databases where horizontal fragments are defined using general select.