network, each under the control of a local DBMS. In a DDBMS a relation may be divided
into a number of fragments that are distributed over a number of sites; fragments may be
replicated. In Section 23.6 we consider query optimization for a DDBMS. Distributed
query optimization is more complex due to the distribution of the data across the sites in
the network. In the distributed environment, as well as local processing costs (that is, CPU
and I/O costs), the speed of the underlying network has to be taken into consideration
when comparing different strategies. In particular, we discuss an extension to the System
R dynamic programming algorithm considered above as well as the query optimization
algorithm from another well-known research project on DDBMSs known as SDD-1.