1. A method of managing stored data objects, comprising: detecting involved attributes of a plurality of stored data objects based on a plurality of object management rules; acquiring the necessary attributes for producing the policy evaluation result with low cost based on selective attribute acquisition and elimination of rule clause evaluations involving attributes determined to have infrequent changes based on historical information, wherein selective attribute acquisition comprises determining expected data storage management costs for each of a plurality of possible orders of attribute acquisition for the detected attributes based on associated attribute acquisition cost for each attribute, and the determined expected data storage management costs is defined based on attribute acquisition costs and failure probabilities, wherein the failure probabilities are continuously refined based on statistics of evaluation results of actual attribute acquisition costs and failures of rule expressions; and executing an object management policy based on analyzing acquisition order of the acquired attributes to manage the plurality of stored data objects at reduced cost for object storage, attribute acquisition and policy evaluation, wherein each attribute is associated with an acquisition cost value.