Currently, MockStage::getStats() returns NULL. This behavior can break unrelated things. Since this stage provides a getCommonStats() method (which will also just return NULL) it would be useful if this stage could just keep around a dummy CommonStats object and use it to generate dummy PlanStageStats objects, something along the lines of:

// add a private class member
CommonStats _commonStats;

// in constructor
 _commonStats("MOCK_STAGE");

// replace the current getCommonStats()
const CommonStats* getCommonStats() {
     return &_commonStats;
}

// replace the current getStats()
const PlanStageStats* MockStage::getStats() {
     return new PlanStageStats(_commonStats, STAGE_MOCK));
}

The same is true of MockStage::getSpecificStats();