Function setop_retrieve_direct

src/backend/executor/nodeSetOp.c:226–333

* ExecSetOp for non-hashed case */

 * ExecSetOp for non-hashed case
 */
static TupleTableSlot *
setop_retrieve_direct(SetOpState *setopstate)
{
	PlanState  *outerPlan;
	SetOpStatePerGroup pergroup;
	TupleTableSlot *outerslot;
	TupleTableSlot *resultTupleSlot;
	ExprContext *econtext = setopstate->ps.ps_ExprContext;

	/*
	 * get state info from node
	 */
	outerPlan = outerPlanState(setopstate);
	pergroup = (SetOpStatePerGroup) setopstate->pergroup;
	resultTupleSlot = setopstate->ps.ps_ResultTupleSlot;

	/*
	 * We loop retrieving groups until we find one we should return
	 */
	while (!setopstate->setop_done)
	{
		/*
		 * If we don't already have the first tuple of the new group, fetch it
		 * from the outer plan.
		 */
		if (setopstate->grp_firstTuple == NULL)
		{
			outerslot = ExecProcNode(outerPlan);
			if (!TupIsNull(outerslot))
			{
				/* Make a copy of the first input tuple */
				setopstate->grp_firstTuple = ExecCopySlotHeapTuple(outerslot);
			}
			else
			{
				/* outer plan produced no tuples at all */
				setopstate->setop_done = true;
				return NULL;
			}
		}

		/*
		 * Store the copied first input tuple in the tuple table slot reserved
		 * for it.  The tuple will be deleted when it is cleared from the
		 * slot.
		 */
		ExecStoreHeapTuple(setopstate->grp_firstTuple,
						   resultTupleSlot,
						   true);
		setopstate->grp_firstTuple = NULL;	/* don't keep two pointers */

		/* Initialize working state for a new input tuple group */
		initialize_counts(pergroup);

		/* Count the first input tuple */
		advance_counts(pergroup,
					   fetch_tuple_flag(setopstate, resultTupleSlot));