1. A method of controlling the robustness level of a query optimizer to facilitate compliance with service level goals and performance predictability in a database system, comprising: receiving, by the database system, a query to be executed; receiving a service level goal associated with the query that specifies a desired processing performance characteristic to be achieved by execution of the query; assigning a confidence threshold parameter having a first value that specifies an optimizer confidence level that execution of a generated query plan for the query will incur a processing cost that will not exceed an estimated processing cost; generating a first query plan in accordance with the confidence threshold parameter; calculating an estimated processing cost of the first query plan that specifies an estimated processing performance characteristic for execution of the first query plan; determining whether the estimated processing cost is less than or equal to the service level goal; lowering the confidence threshold parameter to a second value when the determining determines that the estimated processing cost is not less than or equal to the service level goal; determining whether the second value of the confidence threshold parameter is less than a threshold boundary value; and generating a second query plan in accordance with the second value of the confidence threshold parameter when the determining determines that the second value is not less than the threshold boundary value.