Enumeration of non-isomorphic processing trees

Enumeration of search spaces belonging to join queries, so far comprises large sets of isomorphic processing trees, i.e. trees that can be equalized by only commutative exchange of the input relations to the operators. Since these diierences can be handled eeectively by the costing method it is suucient to focus on the enumeration of non-isomorphic processing trees only 2]. This restriction reduces the size of the search space by factor 2 k , k being the number of operators. In this paper we present a technique that generates all non-isomorphic trees belonging to an arbitrarily shaped query graph. The algorithm generates sequences over the query graph's edges which are simultaneously turned into processing trees. During the incremen-tal expansion of sequence preexes the remaining edges are classiied and only those not leading to isomorphic duplicates are appended. This incremental proceeding causes only little overhead leading to superior performance.