1. A computer-implemented method comprising: maintaining a plurality of generators, each generator configured to generate a sub-plan including instructions to generate execution plans for one or more columns of reports comprising information of an advertising campaign; receiving, by a report execution engine, a report definition specifying one or more columns that include one or more metrics for evaluating the advertising campaign; sending the report definition to a first generator of the plurality of generators, the first generator configured to generate execution plan for at least a column of the report definition; generating, by the first generator, a first sub-plan for processing at least a first column of the report definition by the generator, the first generator incapable of processing a set of remaining columns of the report definition; sending information describing the remaining columns of the report definition to a second generator for processing; receiving a second sub-plan from the second generator, the second sub-plan for processing the remaining columns of the report definition; generating a combined execution plan comprising the first sub-plan and the second sub-plan and an operator combining the first sub-plan and the second sub-plan; executing the combined execution plan to generate results based on the report definition; and sending the generated results for presentation.