2. The method of claim 1 , wherein determining whether the estimated processing cost is less than or equal to the service level goal comprises determining the estimated processing cost is less than or equal to the service level goal, the method further comprising executing the first query plan.
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.