This paper presents a framework for data integration currently under development in the Squirrel project. The framework is based on a special class of mediators, called Squirrel integration mediators. These mediators can support the traditional virtual and materialized approaches, and also hybrids of them. This permits considerable exibility when adapting to diverse data integration environments. In the Squirrel mediators described here, a relation in the integated view can be supported as (a)

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... ully materialized, (b) fully virtual, or (c) partially materialized (i.e., with some attributes materialized and other attributes virtual). In general, (partially) materialized relations of the integrated view are maintained by incremental updates from the source databases. Squirrel mediators provide two approaches for doing this: (1) materialize all needed auxiliary data, so that data sources do not have to be queried when processing the incremental updates; and (2) leave some or all of the auxiliary data virtual, and query selected source databases when processing the incremental update. A central construct used by Squirrel mediators to support integrated views is the notion of \View Decomposition Plan" (VDP). A VDP provides a systematic framework for supporting materialized, virtual, and hybrid relations, including support for (i) retrieval from source databases of virtual data needed to answer queries, and (ii) incremental update of materialized data (including querying of source databases as necessary). VDPs provide a framework for optimizing support for integrated views in a manner reminiscent of query execution plans.