An execution plan of a query for a parallel database system is determined. In one arrangement, the parallel database system is a relational database management system. The execution plan of the query is determined by an optimizer program. Execution plans are inserted into one or more explain tables that are accessible by a visual explain and compare module to display the steps of the execution plan in a graphical user interface. Parallel execution of steps of the execution plan can be depicted in the graphical user interface. Further, plural execution plans for a given query under different conditions may be compared.