PostgreSQL数据库查询执行——PORTAL_ONE_SELECT策略二

亦凉 2022-10-08 15:25 114阅读 0赞

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);
    }