I'm not saying this is a bug, but just curious why param 0 cannot be displayed as the referenced expression. And I find the reason is that in function find_param_referent(), we have the 'in_same_plan_level' flag controlling that if we have emerged from a subplan, i.e. not the same plan level any more, we would not look further for the matching NestLoopParam. Param 0 suits this situation.
And there is a comment there also saying,
/* * NestLoops transmit params to their inner child only; also, once * we've crawled up out of a subplan, this couldn't possibly be * the right match. */
After thinking of this for more time, I still don't see the reason why we cannot display NestLoopParam after we've emerged from a subplan.
It seems these params are from parameterized subqueryscan and their values are supplied by an upper nestloop. These params should have been processed in process_subquery_nestloop_params() that we just add the PlannerParamItem entries to root->curOuterParams, in the form of NestLoopParam, using the same PARAM_EXEC slots.
So I propose the patch attached to remove the 'in_same_plan_level' flag so that we can display NestLoopParam across subplan. Please correct me if I'm wrong.
