Function ExecScan

src/backend/executor/execScan.c:161–271

---------------------------------------------------------------- * ExecScan * * Scans the relation using the 'access method' indicated and * returns the next qualifying tuple. * 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 t

 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecScan(ScanState *node,
		 ExecScanAccessMtd accessMtd,	/* function returning a tuple */
		 ExecScanRecheckMtd recheckMtd)
{
	ExprContext *econtext;
	ExprState  *qual;
	ProjectionInfo *projInfo;

	SIMPLE_FAULT_INJECTOR("before_exec_scan");

	/*
	 * 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;