Abstract: A computer-implemented method is disclosed for executing aggregate queries of composite relationships from an Entity-Relationship (ER) model which sources data from a plurality of heterogeneous and decentralised databases. The method comprises maintaining an ER model comprising a plurality of entities En for which data exists in one or more databases, and one or more relationships rn between those entities. An aggregate query is received relating to a composite relationship cr(r1, r2) involving entities E1, E2 and E3 wherein r1 is a relation between E1 and E2, and wherein r2 is a relation between E2 and E3. The computation of the aggregate query is optimized by selecting an aggregation strategy according to the relation between entities E1, E2 and E3, such that a single table aggregation strategy, a semi-join strategy or a join strategy is selected according to the relation between tables defining entities in the ER model.

Abstract: A computer-implemented method is disclosed for executing aggregate queries of composite relationships from an Entity-Relationship (ER) model which sources data from a plurality of heterogeneous and decentralised databases. The method comprises maintaining an ER model comprising a plurality of entities En for which data exists in one or more databases, and one or more relationships rn between those entities. An aggregate query is received relating to a composite relationship cr(r1, r2) involving entities E1, E2 and E3 wherein r1 is a relation between E1 and E2, and wherein r2 is a relation between E2 and E3. The computation of the aggregate query is optimized by selecting an aggregation strategy according to the relation between entities E1, E2 and E3, such that a single table aggregation strategy, a semi-join strategy or a join strategy is selected according to the relation between tables defining entities in the ER model.