ExecScanFetch

/*
 * -- check interrupts & 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;

	CHECK_FOR_INTERRUPTS();

	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;

		if (scanrelid == 0)
		{
    
			TupleTableSlot *slot = node->ss_ScanTupleSlot;

			/*
			 * This is a ForeignScan or CustomScan which has pushed down a
			 * join to the remote side.  The recheck method is responsible not
			 * only for rechecking the scan/join quals but also for storing
			 * the correct tuple in the slot.
			 */
			if (!(*recheckMtd) (node, slot))
				ExecClearTuple(slot);	/* would not be returned by scan */
			return slot;
		}
		else 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);
}

ExecSeqScan

static TupleTableSlot *
ExecSeqScan(PlanState *pstate)
{
    
	SeqScanState *node = castNode(SeqScanState, pstate);

	return ExecScan(&node->ss,
					(ExecScanAccessMtd) SeqNext,
					(ExecScanRecheckMtd) SeqRecheck);
}

TupIsNull(slot)

/*
 * TupIsNull -- is a TupleTableSlot empty?
 */
#define TupIsNull(slot) \
	((slot) == NULL || (slot)->tts_isempty)

ResetExprContext(econtext)

#define ResetExprContext(econtext) \
	MemoryContextReset((econtext)->ecxt_per_tuple_memory)

MemoryContextReset

/*
 * MemoryContextReset
 *		Release all space allocated within a context and delete all its
 *		descendant contexts (but not the named context itself).
 */
void
MemoryContextReset(MemoryContext context)
{
    
	AssertArg(MemoryContextIsValid(context));

	/* save a function call in common case where there are no children */
	if (context->firstchild != NULL)
		MemoryContextDeleteChildren(context);

	/* save a function call if no pallocs since startup or last reset */
	if (!context->isReset)
		MemoryContextResetOnly(context);
}

ExecScan

/* ----------------------------------------------------------------
 *		Scans the relation using the 'access method' indicated and
 *		returns the next qualifying tuple in the direction specified
 *		in the global variable ExecDirection.
 *		The access method returns the next tuple and ExecScan() is
 *		responsible for checking the tuple returned against the qual-clause.
 *
 *		A 'recheck method' must also be provided that can check an
 *		arbitrary tuple of the relation against any qual conditions
 *		that are implemented internal to the access method.
 *
 *		Conditions:
 *		  -- the "cursor" maintained by the AMI is positioned at the tuple
 *			 returned previously.
 *
 *		Initial States:
 *		  -- the relation indicated is opened for scanning so that the
 *			 "cursor" is positioned before the first qualifying tuple.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecScan(ScanState *node,
		 ExecScanAccessMtd accessMtd,	/* function returning a tuple */
		 ExecScanRecheckMtd recheckMtd)
{
    
	ExprContext *econtext;
	ExprState  *qual;
	ProjectionInfo *projInfo;

	/*
	 * Fetch data from node
	 */
	qual = node->ps.qual;
	projInfo = node->ps.ps_ProjInfo;
	econtext = node->ps.ps_ExprContext;

	/* interrupt checks are in ExecScanFetch */

	/*
	 * If we have neither a qual to check nor a projection to do, just skip
	 * all the overhead and return the raw scan tuple.
	 */
	if (!qual && !projInfo)
	{
    
		ResetExprContext(econtext);
		return ExecScanFetch(node, accessMtd, recheckMtd);
	}

	/*
	 * Reset per-tuple memory context to free any expression evaluation
	 * storage allocated in the previous tuple cycle.
	 */
	ResetExprContext(econtext);

	/*
	 * get a tuple from the access method.  Loop until we obtain a tuple that
	 * passes the qualification.
	 */
	for (;;)
	{
    
		TupleTableSlot *slot;

		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_state.resultslot);
			else
				return slot;
		}

		/*
		 * place the current tuple into the expr context
		 */
		econtext->ecxt_scantuple = slot;

		/*
		 * check that the current tuple satisfies the qual-clause
		 *
		 * check for non-null qual here to avoid a function call to ExecQual()
		 * when the qual is null ... saves only a few cycles, but they add up
		 * ...
		 */
		if (qual == NULL || ExecQual(qual, econtext))
		{
    
			/*
			 * Found a satisfactory scan tuple.
			 */
			if (projInfo)
			{
    
				/*
				 * Form a projection tuple, store it in the result tuple slot
				 * and return it.
				 */
				return ExecProject(projInfo);
			}
			else
			{
    
				/*
				 * Here, we aren't projecting, so just return scan tuple.
				 */
				return slot;
			}
		}
		else
			InstrCountFiltered1(node, 1);

		/*
		 * Tuple fails qual, so free per-tuple memory and try again.
		 */
		ResetExprContext(econtext);
	}
}