1. A method implemented on a general-purpose computing device for discovering and expressing query hints in a database management system, comprising: using the general-purpose computing device to perform the following: input a query having an initial query execution plan; computing a direct acyclic graph for the query; constraining the query using a power hints query language expression that is appended to the query and contains a predicate; computing candidate matches for each operator in the initial execution plan; enumerating possible query execution plans in the directed acyclic graph and filtering out any of the possible query execution plans that do not match the power hints language expression to obtain filtered possible query execution plans; extracting a revised query execution plan that is a revision of the initial query execution plan, the revised query execution plan being one of the filtered possible query execution plans having a lowest cost as determined by a query optimizer cost model that satisfies constraints of the power hints query language expression; displaying the revised query execution plan in a graphical manner to a user; and allowing the user to change the revised query execution plan if the user is not satisfied with the revised query execution plan.