1. A computer-implemented method for verifying correctness of a database system, comprising: receiving a SQL instruction associated with an access path including elements arranged in an order and initial values for those elements, wherein the elements of the associated access path specify operations to implement the SQL instruction and a plurality of values are associated with each element of the associated access path; producing extended access paths for the received SQL instruction from the associated access path by altering the initial values for elements of the associated access path to others of the plurality of associated values, wherein each extended access path includes the same order of elements as the associated access path with a different combination of values for those elements; calculating weights of the extended access paths according to weights of the elements in the extended access paths and historical error information captured from operation of the database system and indicating chances for an error in the database system to be attributed to the associated values of the elements in the extended access paths; executing the SQL instruction by using the extended access paths based on the calculated weights; and verifying correctness of the database system according to a comparison of results from executing the SQL instruction using the extended access paths, wherein an error is present in the database system in response to a difference in results among the extended access paths.