Abstract
We consider the problems of conjunctive query containment and minimization, which are known to be NP-complete, and show that these problems can be solved in polynomial time for the class of acyclic queries. We then generalize the notion of acyclicity and define a parameter called query width that captures the “degree of cyclicity” of a query: in particular, a query is acyclic if and only if its query width is 1. We give algorithms for containment and minimization that run in time polynomial in nk, where n is the input size and k is the query width. These algorithms naturally generalize those for acyclic queries, and are of practical significance because many queries have small query width compared to their sizes. We show that we can obtain good bounds on the query width of Q using the treewidth of the incidence graph of Q. Finally, we apply our containment algorithm to the practically important problem of finding equivalent rewritings of a query using a set of materialized views.
Supported by NSF Award CCR-9357849, with matching funds from IBM, Mitsubishi, Schlumberger Foundation, Shell Foundation, and Xerox Corporation.
Supported by NSF grant IRI-92-23405, ARO grant DAAH04-95-1-0192, and USAF contract F33615-93-1-1339.
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© 1996 Springer-Verlag Berlin Heidelberg
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Chekuri, C., Rajaraman, A. (1996). Conjunctive query containment revisited. In: Afrati, F., Kolaitis, P. (eds) Database Theory — ICDT '97. ICDT 1997. Lecture Notes in Computer Science, vol 1186. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-62222-5_36
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DOI: https://doi.org/10.1007/3-540-62222-5_36
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