1. A method comprising: identifying a plurality of query execution plans that have been designated as acceptable for use by a database server to execute a database command, the database command being a query statement that instructs the database server to perform one or more operations on data stored in a database; persistently maintaining, at the database server, for the database command, baseline data specifying the plurality of query execution plans that have been designated as acceptable for executing the database command, wherein the baseline data includes a plurality of query execution plan outlines, wherein the plurality of query execution plan outlines is a set of data from which the database server reconstructs each query execution plan in the plurality of query execution plans; while persistently maintaining the baseline data, the database server receiving a first request to execute the database command; estimating, by the database server, for each given query execution plan in the plurality of query execution plans, predicted costs for executing the database command in response to the first request according to the given query execution plan; based on the predicted costs for each query execution plan in the plurality of query execution plans, the database server selecting a particular query execution plan from the plurality of query execution plans that have been designated as acceptable for executing the database command; and in response to the first request to execute the database command, the database server executing the database command according to the particular query execution plan; wherein the method is performed by one or more computing devices.