Union of conjunctive queries

  • A conjunctive query with negation (CQ\xac) is of the form: q = ans(x1 xq) ← p1,,pn,n1,,nm, where each pi (resp. ni ) is a positive (resp. negative) subgoal, 1 ≤ n + m, and ans is a special relation (which defines the answer part of the query).
A UCQ is a query of the form q1 ∪ q2 ∪ qm, where each qi is a conjunctive query. For example, the query q = q1 ∪ q2, where q1(x, y) = R(x, z), R(z, y), and q2(x, y) = R(x, z), R(z, w), R(w, y) is a UCQ that returns the endpoints of paths of length 2 or 3.
The class UCQ (Union of Conjunctive Queries) is obtained by adding union to conjunctive queries. A UCQ is a query of the form q1 ∪ q2 ∪ qm, where each qi is a conjunctive query.
The class UCQ (Union of Conjunctive Queries) is obtained by adding union to conjunctive queries. A UCQ is a query of the form q1 ∪ q2 ∪ qm, where each qi is a conjunctive query.

What is the computational complexity of evaluating conjunctive queries?

For the study of the computational complexity of evaluating conjunctive queries, two problems have to be distinguished.
The first is the problem of evaluating a conjunctive query on a relational database where both the query and the database are considered part of the input.


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