The containment problem for REAL conjunctive queries with inequalities

Query containment is a fundamental algorithmic problem in database query processing and optimization. Under set semantics, the query-containment problem for conjunctive queries has long been known to be NP-complete. In real database systems, however, queries are usually evaluated under bag semantics, not set semantics. In particular, SQL queries are evaluated under bag semantics and return multisets as answers, since duplicates are not eliminated unless explicitly requested. The exact complexity of the query-containment problem for conjunctive queries under bag semantics has been an open problem for more than a decade; in fact, it is not even known whether this problem is decidable. Here, we investigate, under bag semantics, the query-containment problem for conjunctive queries with inequalities. It has been previously shown that, under set semantics, this problem is complete for the second level of the polynomial hierarchy. Our main result asserts that, under bag semantics, the query...

• Type: Conference
• Year: 2006
• Where: PODS
• Authors: T. S. Jayram, Phokion G. Kolaitis, Erik Vee
• Keywords: Bag Semantics, Conjunctive Queries, Database, Keywords Conjunctive Queries, PODS 2006