Function gather_getnext

src/backend/executor/nodeGather.c:263–306

* Read the next tuple. We might fetch a tuple from one of the tuple queues * using gather_readnext, or if no tuple queue contains a tuple and the * single_copy flag is not set, we might generate one locally instead. */

 * single_copy flag is not set, we might generate one locally instead.
 */
static TupleTableSlot *
gather_getnext(GatherState *gatherstate)
{
	PlanState  *outerPlan = outerPlanState(gatherstate);
	TupleTableSlot *outerTupleSlot;
	TupleTableSlot *fslot = gatherstate->funnel_slot;
	MinimalTuple tup;

	while (gatherstate->nreaders > 0 || gatherstate->need_to_scan_locally)
	{
		CHECK_FOR_INTERRUPTS();

		if (gatherstate->nreaders > 0)
		{
			tup = gather_readnext(gatherstate);

			if (HeapTupleIsValid(tup))
			{
				ExecStoreMinimalTuple(tup,	/* tuple to store */
									  fslot,	/* slot to store the tuple */
									  false);	/* don't pfree tuple  */
				return fslot;
			}
		}

		if (gatherstate->need_to_scan_locally)
		{
			EState	   *estate = gatherstate->ps.state;

			/* Install our DSA area while executing the plan. */
			estate->es_query_dsa =
				gatherstate->pei ? gatherstate->pei->area : NULL;
			outerTupleSlot = ExecProcNode(outerPlan);
			estate->es_query_dsa = NULL;

			if (!TupIsNull(outerTupleSlot))
				return outerTupleSlot;

			gatherstate->need_to_scan_locally = false;
		}
	}

	return ExecClearTuple(fslot);
}

/*
 * Attempt to read a tuple from one of our parallel workers.