The QE's Pipeline class already contains accumulatePlanSummaryStats() method which greatly resembles PlanExplainerPipeline::getSummaryStats().

Having those methods side-by-side might help to understand the differences in handling the cursor stage and the nReturned counter.

Another benefit of this ticket is to increase the encapsulation of the QE pipeline class by minimizing the number of getStages() calls.

We might also check and document in the code if and why handling the cursor stage differently is necessary.