1. A method for determining a result for a query of data in a probabilistic database, wherein elements of the data are each associated with a probability, a result for the query including a probability for each answer of the result to indicate a relative likelihood that the answer satisfies the query, comprising the steps of: (a) determining if a safe extensional plan defining an order for operators employed in the query can be identified; and (i) if a safe extensional plan can be identified, employing the safe extensional plan to evaluate the query and determine the result that includes a correct probability for each potential answer within the data; and (ii) if a safe extensional plan cannot be identified, selecting an alternative plan that will produce the result with an acceptable error in the probability for each potential answer within the data, and employing the alternative plan to produce the result; and (b) providing the result to a user.