PostgreSQL数据库查询执行—

PostgreSQL数据库查询执行——SeqScan节点执行

news2025/1/9 14:33:55

SeqScan节点代码处于src/backend/executor/nodeSeqscan.c文件中，包含了4个重要函数：ExecInitSeqScan、ExecSeqScan、ExecReScanSeqScan和 ExecEndSeqScan。

ExecInitSeqScan

请添加图片描述
src/backend/executor/nodeSeqscan.c文件中的ExecInitSeqScan函数，其主要包含如下步骤，如上图红色所标出：

创建SeqScanState结构体，关联执行计划树上的SeqScan节点，关联estate
创建expression context
打开scan relation，根据行类型创建合适的槽（PostgreSQL数据库TableAM——HeapAM TupleTableSlot类型中描述了src/backend/access/table/tableam.c向外部模块提供了table_slot_callbacks函数用于获取tableAM创建的表适合的数据tuple存储槽），并将槽关联到estate的es_tupleTable List中
初始化结果类型和投影
初始化child experssions

SeqScanState *ExecInitSeqScan(SeqScan *node, EState *estate, int eflags){
	/* create state structure */
	SeqScanState *scanstate = makeNode(SeqScanState);
	scanstate->ss.ps.plan = (Plan *) node; scanstate->ss.ps.state = estate; scanstate->ss.ps.ExecProcNode = ExecSeqScan;
	/* create expression context for node */
	ExecAssignExprContext(estate, &scanstate->ss.ps);
	/* open the scan relation */
	scanstate->ss.ss_currentRelation = ExecOpenScanRelation(estate, node->scanrelid, eflags);
	/* and create slot with the appropriate rowtype */
	ExecInitScanTupleSlot(estate, &scanstate->ss, RelationGetDescr(scanstate->ss.ss_currentRelation), table_slot_callbacks(scanstate->ss.ss_currentRelation));

	/* Initialize result type and projection. */
	ExecInitResultTypeTL(&scanstate->ss.ps);
	ExecAssignScanProjectionInfo(&scanstate->ss);
	/* initialize child expressions */
	scanstate->ss.ps.qual = ExecInitQual(node->plan.qual, (PlanState *) scanstate);

	return scanstate;
}

ExecSeqScan

ExecSeqScan函数顺序扫描表，返回下一个合适的元组。ExecScan函数会调用SeqNext函数，该函数是真正从表中获取元组的工作函数。

/* ----------------------------------------------------------------
 *		ExecSeqScan(node)
 *		Scans the relation sequentially and returns the next qualifying
 *		tuple.
 *		We call the ExecScan() routine and pass it the appropriate
 *		access method functions.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *ExecSeqScan(PlanState *pstate){
	SeqScanState *node = castNode(SeqScanState, pstate);
	return ExecScan(&node->ss,  (ExecScanAccessMtd) SeqNext,  (ExecScanRecheckMtd) SeqRecheck);
}

SeqNext会做三件事：1. 如果不存在scandesc则创建并初始化一个scandesc（这在顺序扫描中会发生，显而易见的是懒加载，只有真正执行时才创建，但是在并行seqscan中，在Gather节点执行函数ExecGather/ExecGatherMerge中的ExecInitParallelPlanExecSeqScanInitializeDSM函数会调用了TableAM提供的table_beginscan_parallel函数，该函数会初始化ParallelTableScanDescData结构体，并调用TableAM begin_scan接口创建TableScanDesc结构体。）由于SeqNext一次只返回一条可见元组，所以需要一个迭代器，用于记录当前遍历到了哪一个缓存页的哪一条元组，scandesc就是这个迭代器。scandesc是一个HeapScanDesc类型的结构体。2. 调用heap_getnext获取一条可见元组，该函数也是全表遍历的一个核心函数，可以参见HeapAM对TableAM的getnextslot的实现。

static TupleTableSlot *SeqNext(SeqScanState *node){
	/* get information from the estate and scan state */
	TableScanDesc scandesc = node->ss.ss_currentScanDesc;
	EState	   *estate = node->ss.ps.state;
	ScanDirection direction = estate->es_direction;
	TupleTableSlot *slot = node->ss.ss_ScanTupleSlot;

	if (scandesc == NULL) { /* We reach here if the scan is not parallel, or if we're serially executing a scan that was planned to be parallel. */
		scandesc = table_beginscan(node->ss.ss_currentRelation, estate->es_snapshot, 0, NULL);
		node->ss.ss_currentScanDesc = scandesc;
	}

	/* get the next tuple from the table */
	if (table_scan_getnextslot(scandesc, direction, slot)) return slot;
	return NULL;
}