1. A computer-implemented method for use in executing a query in a database system, the method comprising: formulating an initial query-execution plan that identifies an expected path for execution of the query; formulating, prior to an execution of said query according to said initial query-execution plan, an alternative query-execution plan for execution of the query, said alternative query-execution plan differing from the initial query-execution plan from a point that occurs after an intermediate checkpoint in the initial query-execution plan; initiating the execution of the query according to said initial query-execution plan; at said intermediate checkpoint in the initial query-execution plan: concluding that an actual result obtained at the intermediate checkpoint in the initial query-execution plan does not match an expected result; and in response, choosing said alternative query-execution plan for the execution of the query; continuing the execution of the query according to said alternative query-execution plan from said point that occurs after said intermediate checkpoint; and abandoning the execution of the query according to said initial query-execution plan from said point that occurs after said intermediate checkpoint.