1. A method of operating on data from a data source, the data stored in a tangible, non-transitory computer-readable medium of the data source, the operating based on a query that is expressed in accordance with a query language applicable to a relational database, the operating including: receiving a query; identifying a data source based on the query; identifying a requesting system other than a system managing a relational database storing data of the data source; generating, by the requesting system, a request to a query planner based on the query; providing, by the requesting system, the request to the query planner; receiving a query plan generated by the query planner based on the request, the query plan including a description of one or more steps to be performed by a system managing a relational database; generating a data structure instantiating a dataflow graph that includes: a first node that represents at least one operation to be executed, the first node associated with information usable by an executing system to invoke executable program code to perform the at least one operation, the first node associated with information usable by an executing system to make data available to the program code, and the at least one operation chosen based on a step described by the query plan, and at least one link that represents output data of an operation of the first node being provided as input data to an operation of a second node of the dataflow graph; receiving data from the data source; identifying an executing system other than a system managing a relational database storing data of the data source; and executing, on the identified executing system, program code based on the dataflow graph.