Query Optimization by Predicate Move-Around

A new type of optimization, called predicate move-around, ia introduced. It is shown how this optimization ‘considerably improvea the efficiency of evaluating SQL queries that have query graphs with a large number of query blocks (which ie a typical situation when queries are defined in terms of multiple views and subqueries). Predicate move-around works by moving predicates across query blocks (in the query graph) that cannot be merged into one block. Predicate move-around is a generalization of and has many advantages over the traditional predicate pushdotin. One key advantage arises from the fact that predicate move-around precedes pushdown by pulling predicates up the query graph. As a result, predicates that appear in the query in one part of the graph can be moved around the graph and applied alao in other parts of graph. Moreover, predicate movearound optimization can move a wider class of predicates in a wider class of queries aa compared to the standard predicate pushdown techniques. In addition to the usual comparison and arithmetic predicates, other predicates that can be moved around are the EXISTS and HOT EXISTS clauses, the EXCEPT clause, and functional dependencies. The proposed optimization can also move predicates through aggregation. Moreover, the method can also infer new predicates when existing predicates are moved through aggregation or when certain functional dependencies are known to hold. Finally, the predicate move-around algorithm is easy to implement on top of existing query optimizers.