Described is a constraint language and related technology by which complex constraints may be used in selecting configurations for use in physical database design tuning. The complex constraint (or constraints) is processed, e.g., in a search framework, to determine and output at least one configuration that meets the constraint, e.g., a best configuration found before a stopping condition is met. The search framework processes a current configuration into candidate configurations, including by searching for candidate configurations from a current configuration based upon a complex constraint, iteratively evaluating a search space until a stopping condition is satisfied, using transformation rules to generate new candidate configurations, and selecting a best candidate configuration. Transformation rules and pruning rules are applied to efficiently perform the search. Constraints may be specified as assertions that need to be satisfied, or as soft assertions that come close to satisfying the constraint.