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.
8. The method of claim 1 , wherein query plan formation data includes an indication that a particular query rewrite optimization failed to eliminate a particular operator from the query plan comprising: in response to determining that a particular query rewrite optimization failed to eliminate an operator from the query plan, determining to add to the query plan display output the indication that the particular query rewrite optimization failed and a reason the particular query rewrite optimization failed to eliminate an operator from the query plan.