Managing Semantic Heterogeneity in Databases: A Theoretical Perspective

Modern database management systems essentially solve the problem of accessing and managing large volumes of related data on a single platform, or on a cluster of tightly­coupled platforms. But many problems remain when two or more databases need to work together. A fundamental problem is raised by semantic heterogeneity -- the fact that data du­ plicated across multiple databases is represented differently in the underlying database schemas. This tutorial describes fundamental problems raised by semantic heterogeneity and surveys theoretical frameworks that can provide solutions for them. The tutorial considers the following topics: (1) repre­ sentative architectures for supporting database interoperation; (2) notions for comparing the "information capacity" of database schemas; (3) providing support for read­only integrated views of data, including the virtual and materialized approaches; (4) providing support for read­write integrated views of data, including the issue of workflows on heterogeneous databases; and (5) research and tools for accessing and effectively using meta­data, e.g., to identify the relationships between schemas of different databases

Admittance and Nonlinear Transport in Quantum Wires, Point Contacts, and Resonant Tunneling Barriers

We present a discussion of the admittance (ac-conductance) and nonlinear I-V-characteristic for a number of mesoscopic conductors. Our approach is based on a generalization of the scattering approach which now includes the effects of the (long-range) Coulomb interaction. We discuss the admittance of a wire with an impurity and with a nearby gate. We extend a discussion of the low-frequency admittance of a quantum point contact to investigate the effects of the gates used to form the contact. We discuss the nonlinear I-V characteristic of a resonant double barrier structure and discuss the admittance for the double barrier for a large range of frequencies. Our approach emphasizes the overall conservation of charge (gauge invariance) and current conservation and the resulting sum rules for the admittance matrix and nonlinear transport coefficients