Declarative Debugging of Wrong and Missing Answers for SQL Views

This paper presents a debugging technique for diagnosing errors in SQL views. The debugger allows the user to specify the error type, indicating if there is either a missing answer (a tuple was expected but it is not in the result) or a wrong answer (the result contains an unexpected tuple). This information is employed for slicing the associated queries, keeping only those parts that might be the cause of the error. The validity of the results produced by sliced queries is easier to determine, thus facilitating the location of the error. Although based on the ideas of declarative debugging, the proposed technique does not use computation trees explicitly. Instead, the logical relations among the nodes of the trees are represented by logical clauses that also contain the information extracted from the specific questions provided by the user. The atoms in the body of the clauses correspond to questions that the user must answer in order to detect an incorrect relation. The resulting logic program is executed by selecting at each step the unsolved atom that yields the simplest question, repeating the process until an erroneous relation is detected. Soundness and completeness results are provided. The theoretical ideas have been implemented in a working prototype included in the Datalog system DES.