Visual query systems (VQSs) are query systems for databases that use visual representations to depict the domain of interest and express related requests. VQSs can be seen as an evolution of query languages adopted into database management systems; they are designed to improve the effectiveness of the human-computer communication. Thus, their most important features are those that determine the nature of the human-computer dialogue. In order to survey and compare existing VQSs used for querying traditional databases, we first introduce a classification based on such features, namely the adopted visual representations and the interaction strategies. We then identify several user types and match the VQS classes against them, in order to understand which kind of system may be suitable for each kind of user. We also report usability experiments which support our claims. Finally, some of the most important open problems in the VQS area are described. 1.

Mobile computers and wireless networks are emerging technologies that will soon be available to a wide variety of computer users. Unlike earlier generations of laptop computers, the new generation of mobile computers can be an integrated part of a distributed computing environment, one in which users change physical location frequently. The result is a new computing paradigm, nomadic computing. This paradigm will affect the design of much of our current systems software, including that of database systems. This paper discusses in some detail the impact of nomadic computing on a number of traditional database system concepts. In particular, we point out how the reliance on short-lived batteries changes the cost assumptions underlying query processing. In these systems, power consumption competes with resource utilization in the definition of cost metrics. We also discuss how the likelihood of temporary disconnection forces consideration of alternative transaction processing pr...

this paper, as well as to Gerd Westerman and Martin Erwig for interesting discussions and comments on a draft version. Thanks to Boris Pasternak for patiently listening and frequently advocating different points of view. References

Computer-based personal information service is evolving beyond simple applications such as retrieval of phone numbers to include interaction with large, geographically distributed information bases. Concurrently, small, pen-based, mobile computers are becoming the machine of choice for personal computing. These two trends place conflicting demands on the design of database interfaces. The latter trend suggests simple interfaces that are easy-to-use, avoid keyboard use, and are suited for the small screens and small (relatively speaking) memory sizes of mobile machines. The former trend, however, suggests an increased sophistication in database interfaces, so as to provide access to the larger databases that are now part of a personal information service. We describe a pen-based graphical database language that begins to combine these conflicting demands for simplicity and sophistication. We compare this language with previous work on graphical user interfaces designed for workstations...