Function ExecScanSubPlan

src/backend/executor/nodeSubplan.c:234–483

* ExecScanSubPlan: default case where we have to rescan subplan each time */

 * ExecScanSubPlan: default case where we have to rescan subplan each time
 */
static Datum
ExecScanSubPlan(SubPlanState *node,
				ExprContext *econtext,
				bool *isNull)
{
	SubPlan    *subplan = node->subplan;
	PlanState  *planstate = node->planstate;
	SubLinkType subLinkType = subplan->subLinkType;
	MemoryContext oldcontext;
	TupleTableSlot *slot;
	Datum		result;
	bool		found = false;	/* true if got at least one subplan tuple */
	ListCell   *pvar;
	ListCell   *l;
	ArrayBuildStateAny *astate = NULL;

	/*
	 * MULTIEXPR subplans, when "executed", just return NULL; but first we
	 * mark the subplan's output parameters as needing recalculation.  (This
	 * is a bit of a hack: it relies on the subplan appearing later in its
	 * targetlist than any of the referencing Params, so that all the Params
	 * have been evaluated before we re-mark them for the next evaluation
	 * cycle.  But in general resjunk tlist items appear after non-resjunk
	 * ones, so this should be safe.)  Unlike ExecReScanSetParamPlan, we do
	 * *not* set bits in the parent plan node's chgParam, because we don't
	 * want to cause a rescan of the parent.
	 */
	if (subLinkType == MULTIEXPR_SUBLINK)
	{
		EState	   *estate = node->parent->state;

		foreach(l, subplan->setParam)
		{
			int			paramid = lfirst_int(l);
			ParamExecData *prm = &(estate->es_param_exec_vals[paramid]);

			prm->execPlan = node;
		}
		*isNull = true;
		return (Datum) 0;
	}

	/* Initialize ArrayBuildStateAny in caller's context, if needed */
	if (subLinkType == ARRAY_SUBLINK)
		astate = initArrayResultAny(subplan->firstColType,
									CurrentMemoryContext, true);

	/*
	 * We are probably in a short-lived expression-evaluation context. Switch
	 * to the per-query context for manipulating the child plan's chgParam,
	 * calling ExecProcNode on it, etc.
	 */
	oldcontext = MemoryContextSwitchTo(econtext->ecxt_per_query_memory);

	/*
	 * Set Params of this plan from parent plan correlation values. (Any
	 * calculation we have to do is done in the parent econtext, since the
	 * Param values don't need to have per-query lifetime.)