PortalRunSelect函数在 PORTAL_ONE_SELECT策略下执行portal查询，在 PORTAL_ONE_RETURNING、PORTAL_ONE_MOD_WITH 和 PORTAL_UTIL_SELECT 情况下从已完成的 HoldStore 中提取数据时也适用。
forward：控制simple N-rows-forward-or-backward情况
count <= 0 被解释为无操作：目标启动和关闭，但没有其他任何事情发生。 此外，count == FETCH_ALL 被解释为“所有行”。 （参见 FetchStmt.howMany）
返回处理的行数（适用于结果标签）

static uint64 PortalRunSelect(Portal portal, bool forward, long count, DestReceiver *dest) {
	QueryDesc  *queryDesc;
	ScanDirection direction;
	uint64		nprocessed;
	/* NB: queryDesc will be NULL if we are fetching from a held cursor or a completed utility query; can't use it in that path. */
	queryDesc = portal->queryDesc;
	/* Caller messed up if we have neither a ready query nor held data. */
	Assert(queryDesc || portal->holdStore);
	/* Force the queryDesc destination to the right thing.  This supports MOVE, for example, which will pass in dest = DestNone.  This is okay to change as long as we do it on every fetch.  (The Executor must not assume that dest never changes.) */
	if (queryDesc) queryDesc->dest = dest;

	if (forward){
		if (portal->atEnd || count <= 0){
			direction = NoMovementScanDirection;
			count = 0;			/* don't pass negative count to executor */
		}
		else
			direction = ForwardScanDirection;
		/* In the executor, zero count processes all rows */
		if (count == FETCH_ALL)
			count = 0;
		if (portal->holdStore)
			nprocessed = RunFromStore(portal, direction, (uint64) count, dest);
		else{
			PushActiveSnapshot(queryDesc->snapshot);
			ExecutorRun(queryDesc, direction, (uint64) count, portal->run_once);
			nprocessed = queryDesc->estate->es_processed;
			PopActiveSnapshot();
		}
		if (!ScanDirectionIsNoMovement(direction)){
			if (nprocessed > 0)
				portal->atStart = false;	/* OK to go backward now */
			if (count == 0 || nprocessed < (uint64) count)
				portal->atEnd = true;	/* we retrieved 'em all */
			portal->portalPos += nprocessed;
		}
	}else{
		if (portal->cursorOptions & CURSOR_OPT_NO_SCROLL)
			ereport(ERROR, (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE), errmsg("cursor can only scan forward"), errhint("Declare it with SCROLL option to enable backward scan.")));
		if (portal->atStart || count <= 0){
			direction = NoMovementScanDirection;
			count = 0;			/* don't pass negative count to executor */
		}
		else
			direction = BackwardScanDirection;
		/* In the executor, zero count processes all rows */
		if (count == FETCH_ALL)
			count = 0;
		if (portal->holdStore)
			nprocessed = RunFromStore(portal, direction, (uint64) count, dest);
		else{
			PushActiveSnapshot(queryDesc->snapshot);
			ExecutorRun(queryDesc, direction, (uint64) count, portal->run_once);
			nprocessed = queryDesc->estate->es_processed;
			PopActiveSnapshot();
		}
		if (!ScanDirectionIsNoMovement(direction))
		{
			if (nprocessed > 0 && portal->atEnd){
				portal->atEnd = false;	/* OK to go forward now */
				portal->portalPos++;	/* adjust for endpoint case */
			}
			if (count == 0 || nprocessed < (uint64) count){
				portal->atStart = true; /* we retrieved 'em all */
				portal->portalPos = 0;
			}
			else
				portal->portalPos -= nprocessed;
		}
	}
	return nprocessed;
}

该函数主要由forward分为两个部分，用于确定要进入的方向，并检查我们是否已经在该方向上可用元组的末尾。 如果是这样，请将方向设置为 NoMovement 以避免尝试获取任何元组。 （此检查存在是因为如果它们已经返回 NULL 一次，则并非所有计划节点类型都可以再次调用。）然后调用执行程序（我们不能跳过此操作，因为目标需要查看设置和关闭，即使没有 元组可用）。 最后，根据检索到的元组数量更新portal位置状态。
如果forward为true时，方向为Forward，判定atEnd或count是否为负或0，主要是不能传输负值给执行器。如果是通过holdStore取值，则执行RunFromStore，否则执行ExecutorRun。

		if (portal->atEnd || count <= 0){
			direction = NoMovementScanDirection;
			count = 0;			/* don't pass negative count to executor */
		}
		else
			direction = ForwardScanDirection;
		/* In the executor, zero count processes all rows */
		if (count == FETCH_ALL)
			count = 0;
		if (portal->holdStore)
			nprocessed = RunFromStore(portal, direction, (uint64) count, dest);
		else
		{
			PushActiveSnapshot(queryDesc->snapshot);
			ExecutorRun(queryDesc, direction, (uint64) count, portal->run_once);
			nprocessed = queryDesc->estate->es_processed;
			PopActiveSnapshot();
		}
		if (!ScanDirectionIsNoMovement(direction))
		{
			if (nprocessed > 0)
				portal->atStart = false;	/* OK to go backward now */
			if (count == 0 || nprocessed < (uint64) count)
				portal->atEnd = true;	/* we retrieved 'em all */
			portal->portalPos += nprocessed;
		}

如果forward为fasle时，方向为Backward，判定atStart或count是否为负或0，主要是不能传输负值给执行器。如果是通过holdStore取值，则执行RunFromStore，否则执行ExecutorRun。

		if (portal->atStart || count <= 0){
			direction = NoMovementScanDirection;
			count = 0;			/* don't pass negative count to executor */
		}else
			direction = BackwardScanDirection;
		/* In the executor, zero count processes all rows */
		if (count == FETCH_ALL)
			count = 0;
		if (portal->holdStore)
			nprocessed = RunFromStore(portal, direction, (uint64) count, dest);
		else{
			PushActiveSnapshot(queryDesc->snapshot);
			ExecutorRun(queryDesc, direction, (uint64) count, portal->run_once);
			nprocessed = queryDesc->estate->es_processed;
			PopActiveSnapshot();
		}
		if (!ScanDirectionIsNoMovement(direction)){
			if (nprocessed > 0 && portal->atEnd){
				portal->atEnd = false;	/* OK to go forward now */
				portal->portalPos++;	/* adjust for endpoint case */
			}
			if (count == 0 || nprocessed < (uint64) count){
				portal->atStart = true; /* we retrieved 'em all */
				portal->portalPos = 0;
			}
			else
				portal->portalPos -= nprocessed;
		}

ExecutorRun函数

void ExecutorRun(QueryDesc *queryDesc, ScanDirection direction, uint64 count, bool execute_once) {
	if (ExecutorRun_hook)
		(*ExecutorRun_hook) (queryDesc, direction, count, execute_once);
	else
		standard_ExecutorRun(queryDesc, direction, count, execute_once);
}

void standard_ExecutorRun(QueryDesc *queryDesc, ScanDirection direction, uint64 count, bool execute_once){
	EState	   *estate;
	CmdType		operation;
	DestReceiver *dest;
	bool		sendTuples;
	MemoryContext oldcontext;
	/* sanity checks */
	Assert(queryDesc != NULL);
	estate = queryDesc->estate;
	Assert(estate != NULL);
	Assert(!(estate->es_top_eflags & EXEC_FLAG_EXPLAIN_ONLY));
	/* Switch into per-query memory context */
	oldcontext = MemoryContextSwitchTo(estate->es_query_cxt);
	/* Allow instrumentation of Executor overall runtime */
	if (queryDesc->totaltime)
		InstrStartNode(queryDesc->totaltime);
	/* extract information from the query descriptor and the query feature. */
	operation = queryDesc->operation;
	dest = queryDesc->dest;
	/* startup tuple receiver, if we will be emitting tuples */
	estate->es_processed = 0;
	sendTuples = (operation == CMD_SELECT || queryDesc->plannedstmt->hasReturning);
	if (sendTuples)
		dest->rStartup(dest, operation, queryDesc->tupDesc);
	/* run plan */
	if (!ScanDirectionIsNoMovement(direction)){
		if (execute_once && queryDesc->already_executed)
			elog(ERROR, "can't re-execute query flagged for single execution");
		queryDesc->already_executed = true;
		ExecutePlan(estate, queryDesc->planstate, queryDesc->plannedstmt->parallelModeNeeded, operation, sendTuples, count, direction, dest, execute_once);
	}
	/* shutdown tuple receiver, if we started it */
	if (sendTuples)
		dest->rShutdown(dest);
	if (queryDesc->totaltime)
		InstrStopNode(queryDesc->totaltime, estate->es_processed);
	MemoryContextSwitchTo(oldcontext);
}