PostgreSQL在何处处理 sql查询之三十九

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)