A tagging tree that defines a hierarchical output data structure and relational database query operations that retrieve the data to be included in each hierarchical section is accepted and processed. A query optimizer analyzes the tagging tree to determine database query operations that can be combined into a single query command. An outer-union query command is generated to allow efficient combination of multiple query commands, some of which are predicates to others. The tagging tree is modified to replace the database queries that were combined into the outer-union query to extract data from the results set produced by the outer-union query as opposed to submitting queries to the database manager. The optimized queries can be reused an arbitrary number of times to leverage the optimizing resources that were used in their creation.