Method and apparatus for efficiently evaluating exists and not exists

Abstract

A method for transforming database queries that have EXISTS-equivalent subqueries (i.e. subqueries that are semantically equivalent to EXISTS or NOT EXISTS subqueries) is disclosed. Logical expressions are removed from a WHERE CLAUSE in the original EXISTS-equivalent subquery to produce a revised subquery. For each of the logical expressions that are removed, a substitute query component is generated based on whether (1) the logical expression contains any reference to any table in a FROM-list in the EXISTS-equivalent subquery and (2) the original EXISTS-equivalent subquery is equivalent to an EXISTS or a NOT EXISTS subquery. The transformed query is created by replacing the original EXISTS-equivalent subquery with the revised subquery and the query components that were generated for each of removed logical expressions. Some of the substitute query components, including the revised subquery, may be merged based on knowledge of access paths. These transformation techniques may be applied after converting the EXISTS-equivalent subquery to an EXISTS or NOT EXISTS subquery. Alternatively, the techniques may be applied directly to queries that contain EXISTS-equivalent subqueries without converting the EXISTS-equivalent subqueries to EXISTS or NOT EXISTS subqueries. The transformed queries produced by this technique may be processed more efficiently by database applications than the original queries.