Abstract

A sequence of interleaved user transactions in a database system may not be 8zis, i.e., equivalent to some sequential execution of the individual transactions. Using a simle transaction model we show that recognizing the transaction histories which are serializable is an NP- complete problem. We therefore introduce several efficiently recognizable j9gsses of the class of serializable histories most of thee S.b- classes correspond to serializability principles existing in the literature and used in practice. We also propose two new principles which subsume all previously known ones. We give necessary and sufficient conditions for a class of histories to be the output of an efficient history scheduler these conditions imply that there can be no efficient scheduler that outputs all of serializable histories,.and also that all subclasses of serializable histories studied above have an efficient scheduler. Finally, we show how our results can be extended to far more general transaction models,'to transactions with partly interpreted functions, and to distributed database syst%.

Citations