19. A system including one or more processors for 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 system configured to execute operations for: receiving a query; identifying a data source based on the query, the data source being associated with a system managing the data source; identifying an executing system other than a system managing a relational database; generating a request to a query planner based on the query; providing 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, based on the query plan, 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 operation, the first node associated with information usable by an executing system to make data available to the program code, and the operation being chosen based on a first step described by the query plan, and a second node associated with information usable by an executing system to invoke executable program code that causes a request to perform at least one operation chosen based on a second step described by the query plan to be transmitted to the system managing the data source; receiving data from the data source; and executing, on the identified executing system, program code based on the dataflow graph.
22. The system of claim 19 in which the second node is associated with information usable by an executing system to receive the data from the data source.