接前面，这次重点分析 ExecScan:
其for 循环内部：
for (;;) { TupleTableSlot *slot; CHECK_FOR_INTERRUPTS(); slot = ExecScanFetch(node, accessMtd, recheckMtd); /* * if the slot returned by the accessMtd contains NULL, then it means * there is nothing more to scan so we just return an empty slot, * being careful to use the projection result slot so it has correct * tupleDesc. */ if (TupIsNull(slot)) { if (projInfo) return ExecClearTuple(projInfo->pi_slot); else return slot; } ... }
接着来看 ExecScanFetch:
/* * ExecScanFetch -- fetch next potential tuple * * This routine is concerned with substituting a test tuple if we are * inside an EvalPlanQual recheck. If we aren't, just execute * the access method's next-tuple routine. */ static inline TupleTableSlot * ExecScanFetch(ScanState *node, ExecScanAccessMtd accessMtd, ExecScanRecheckMtd recheckMtd) { EState *estate = node->ps.state; if (estate->es_epqTuple != NULL) { /* * We are inside an EvalPlanQual recheck. Return the test tuple if * one is available, after rechecking any access-method-specific * conditions. */ Index scanrelid = ((Scan *) node->ps.plan)->scanrelid; Assert(scanrelid > 0); if (estate->es_epqTupleSet[scanrelid - 1]) { TupleTableSlot *slot = node->ss_ScanTupleSlot; /* Return empty slot if we already returned a tuple */ if (estate->es_epqScanDone[scanrelid - 1]) return ExecClearTuple(slot); /* Else mark to remember that we shouldn't return more */ estate->es_epqScanDone[scanrelid - 1] = true; /* Return empty slot if we haven't got a test tuple */ if (estate->es_epqTuple[scanrelid - 1] == NULL) return ExecClearTuple(slot); /* Store test tuple in the plan node's scan slot */ ExecStoreTuple(estate->es_epqTuple[scanrelid - 1], slot, InvalidBuffer, false); /* Check if it meets the access-method conditions */ if (!(*recheckMtd) (node, slot)) ExecClearTuple(slot); /* would not be returned by scan */ return slot; } } /* * Run the node-type-specific access method function to get the next tuple */ return (*accessMtd) (node); }
上述的 ＜(estate->es_epqTuple != NULL)＞ 条件未能得到满足，所以
直接奔这个去了： return (*accessMtd) (node);
前面已经知道，因为没有index，所以就执行了：static TupleTableSlot * SeqNext(SeqScanState *node)
本文转自健哥的数据花园博客园博客，原文链接：http://www.cnblogs.com/gaojian/archive/2013/05/31/3110493.html，如需转载请自行联系原作者
