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);elsereturn slot;}        ...}

/** 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);
}