Databases Illuminated, 4th Edition

12.7 Distributed Query Processing

To process a query expressed in a high-level language such as SQL, it is necessary to transform the query into a low-level representation such as relational algebra before execution. There are various methods of transforming relational algebra expressions into equivalent but more efficient ones, and several methods for estimating the cost of processing queries for different relational algebra operations, which are discussed in Chapter 10. The dominant factor in the cost estimates is the number of disk accesses required. A distributed environment requires that we also consider the cost of transmitting data over the network. If the network is relatively slow, this factor can become the dominant cost factor. If ...