1. A method of improving the robustness of a query optimizer and the predictability of the performance of execution plans in a database system, comprising: receiving, by an optimizer of the database system, a first query to be executed; obtaining, by the optimizer, a confidence threshold parameter associated with the first query; generating a query execution plan for the first query utilizing an optimizer confidence value compliant with the confidence threshold parameter; obtaining, by a runtime query execution system, an error exception threshold associated with the first query; determining a query execution cost of the query execution plan exceeds an optimizer estimated cost of the query execution plan by an amount that exceeds the error exception threshold; determining an exception count equals a maximum exception count parameter; and generating an adjusted confidence threshold parameter responsive to determining the exception count equals the maximum exception count parameter.