1. A method comprising: generating a query plan for executing a query conforming to a database language; wherein generating a query plan includes: incorporating one or more operators into the query plan, wherein each of the one or more operators comprises a primitive operation; and generating query plan formation data that specifies one or more factors causing the incorporating of said one or more operators in the query plan; based on one or more criteria, generating query plan display output that includes for each operator of said one or more operators: output identifying said each operator; and output identifying a factor of said one or more factors causing the incorporating of said each operator in the query plan; and wherein the method is performed by one or more computing devices.