Bag Semantics Query Containment: The CQ vs. UCQ Case and Other Stories

Query Containment Problem (QCP) is a fundamental decision problem in query processing and optimization. While QCP has for a long time been completely understood for the case of set semantics, decidability of QCP for conjunctive queries under multi-set semantics (QCPbag(CQ)) remains one of the most intriguing open problems in database theory. Certain effort has been put, in last 30 years, to solve this problem and some decidable special cases of QCPbag(CQ) were identified, as well as some undecidable extensions, including QCPbag(UCQ). In this paper we introduce a new technique which produces, for a given UCQ {\Phi}, a CQ {\phi} such that the application of {\phi} to a database D is, in some sense, an approximation of the application of {\Phi} to D. Using this technique we could analyze the status of QCPbag when one of the queries in question is a CQ and the other is a UCQ, and we reached conclusions which surprised us a little bit. We also tried to use this technique to translate the known undecidability proof for QCPbag(UCQ) into a proof of undecidability of QCPbag(CQ). And, as you are going to see, we got stopped just one infinitely small {\epsilon} before reaching this ultimate goal.