Equivalence of nested queries with mixed semantics

Abstract

We consider the problem of deciding query equivalence for a conjunctive language in which queries output complex objects composed from a mixture of nested, unordered collection types. Using an encoding of nested objects as flat relations, we translate the problem to deciding the equivalence between encodings output by relational conjunctive queries. This encoding equivalence cleanly unifies and generalizes previous results for deciding equivalence of conjunctive queries evaluated under various processing semantics. As part of our characterization of encoding equivalence, we define a normal form for encoding queries and contend that this normal form offers new insight into the fundamental principles governing the behaviour of nested aggregation.