Databases Reference
In-Depth Information
Among the works related to query processing in mobile computing we
can mention [3740]. In [37] those authors present how to deal with queries
with location constraints, that is, queries that involve the individual locations
of users. Because the location of a user is not exact, it is expensive, in terms
of communication costs, to find out the missing information necessary to
answer queries with location constraints. The rest of the mentioned works try
to provide solutions to more specific problems. In [38, 39] the authors try to
facilitate traveling by providing updated information on traffic conditions,
weather, available parking, shortest distances, emergency services, and so
forth. Needed data can be obtained by making specific requests (pull based)
and by data dissemination or broadcasting (push based). In [40] a Web
information system for a mobile wireless computing environment is
described. The Web is extended to allow documents to refer and react to
changing contextual information, like current location in a wireless network.
It introduces the concept of dynamic URLs (which allow to return different
documents or execute different commands depending on dynamic environ-
ment variables) and the concept of active documents, which automatically
update their contents in response to changes in the users mobile context.
10.6.4
Caching
As mentioned previously, query optimization methods try to minimize CPU,
I/O, and communication costs; in the mobile distributed context, the com-
munication costs may be particularly important. It is accepted that applying
caching techniques for query processing can reduce communication costs
dramatically. However, it is more difficult to apply caching techniques in the
mobile context because cache contents may change rapidly or become out-
of-date due to mobility; in addition, because of disconnections of the MU,
updates to the cache memory may not be sent. The authors of [18] present
several techniques related to caching, such as prefetching, replacement strate-
gies, and consistency of the cache memory, used in combination with broad-
casting techniques. The idea is to send by broadcast channel some data that
may be needed in the future (prefetching) or data that have become
inconsistent.
10.6.5
Database Interfaces
The limited screen sizes of many mobile computers have motivated the
development of new interfaces for them and, in particular, the design of new
DB interfaces for mobile computers. In [41] there appears a query processing
