As queries grow increasingly complex and large data sets are becoming prevalent, Parallel Query Processing, 
database sizes grow dramatically particularly in Decision Support Systems (DSS) , and OnLine
Analytic Processing Systems ( OIAP) which have recently emerged as important
database applications. In these systems, performance is a critical issue and speeding up the
system has always been an objective but the processing power of individual processors
can only handle a small fraction of current applications. As a result, parallel processing is
exploited to improve database systems performance. In the thesis we focus on relational
database systems and study skew characteristics and their effects on parallel query
processing.