The notion of SQL assertion was introduced, in SQL-92 standard, to define general constraints over a relational database. They can be used, for instance, to specify cross-row constraints or multitable check constraints. However, up to now, none of the current relational database management systems (RDBMSs) support SQL assertions due to the difficulty of providing an efficient solution.
To implement SQL assertions efficiently, the RDBMs require an incremental checking mechanism. I.e., given an assertion, the RDBMS should revalidate it only when a transaction changes data in a manner that could violate it, and only for the affected data. Some years ago, the deductive database community provided several incremental checking methods, however, their results could not get into practice in RDBMS.
In this paper, we propose an approach to efficiently implement SQL assertions in an RDBMS through an incremental revalidation technique. Such an approach is compatible with any RDBMS since it is fully based on standard SQL concepts (tables, triggers, and procedures). Our proposal uses and extends the Event Rules, an existing proposal for incremental checking in deductive databases. This extension is required to handle distributive aggregates, which pushes the expressiveness of the handled SQL assertions beyond first-order constraints. Moreover, we exploit this extension to improve the treatment of constraints involving existential variables, which are a very common kind of constraints difficult and expensive to handle. Finally, we show the efficiency of our approach through some experiments, and we formally prove its soundness and completeness.
