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Some results on the containment and minimization of (in)equality queries. (English)
Inf. Process. Lett. 50, No.5, 259-267 (1994).
Summary: The containment and minimization of conjunctive queries are two basic problems of query optimization in relational and deductive databases. For equality queries, the problems are NP-complete due to the “homomorphism property”. For (in)equality queries, however, the containment problem has recently been proved to be \$Π\sp p\sb 2\$-complete, while the minimization is virtually unknown. In this paper, we identify subclasses of (in)equality queries which have the homomorphism property and a similar minimization procedure to the one for equality queries. We also give interesting examples to show that queries outside these classes do not have the property.