Why Not Match: On Explanations of Event Pattern Queries

ABSTRACTQueries over event data are posed in a form of event patterns, for example, to retrieve the flights from IAH to LGA without a stopover. If the expected answer is not returned, one may ask why not, also known as explanations of non-answers. Analogous to the relational data, the explanations over event data lie in two aspects. (1) The pattern consistency explanation indicates that the patterns specified in the query are wrong (inconsistent), that is, there exists no tuple of events that can match the query. (2) The timestamp modification explanation speculates that the instance of event tuple is incorrect, for example, the timestamps of some events are imprecise and need modification. To the best of our knowledge, this is the first study on explaining non-answers over event data. We prove that both explanation problems are NP-complete. By encoding event patterns as a novel notation, we identify the special cases that can be efficiently solved or approximated. General cases are addressed by utilizing the solutions of special cases. Extensive experiments over real and synthetic datasets demonstrate both effectiveness and efficiency of our proposal.