OR Expansion

Wieder ein Blog-Artikel des Optimizer-Teams zu einer internen Rewrite-Strategie des cbo, der OR Expansion:

OR expansion is a transformation that can be used to optimize disjunctive queries (queries that contain OR clauses). The basic idea in OR expansion is to transform a query containing disjunctions into the form of a UNION ALL query of two or more branches. This is done by splitting the disjunction into its components and associating each component with a branch of a UNION ALL query.

There are many reasons for performing OR expansion. It can enable more efficient access paths (index accesses, partition pruning), open up alternative join methods (avoid Cartesian product).

Klingt recht harmlos und scheint das auch zu sein.