Function ExecLimit_guts

src/backend/executor/nodeLimit.c:43–349

---------------------------------------------------------------- * ExecLimit * * This is a very simple node which just performs LIMIT/OFFSET * filtering on the stream of tuples returned by a subplan. * ---------------------------------------------------------------- */

 * ----------------------------------------------------------------
 */
static TupleTableSlot *			/* return: a tuple or NULL */
ExecLimit_guts(PlanState *pstate)
{
	LimitState *node = castNode(LimitState, pstate);
	ExprContext *econtext = node->ps.ps_ExprContext;
	ScanDirection direction;
	TupleTableSlot *slot;
	PlanState  *outerPlan;

	CHECK_FOR_INTERRUPTS();

	/*
	 * get information from the node
	 */
	direction = node->ps.state->es_direction;
	outerPlan = outerPlanState(node);

	/*
	 * The main logic is a simple state machine.
	 */
	switch (node->lstate)
	{
		case LIMIT_INITIAL:

			/*
			 * First call for this node, so compute limit/offset. (We can't do
			 * this any earlier, because parameters from upper nodes will not
			 * be set during ExecInitLimit.)  This also sets position = 0 and
			 * changes the state to LIMIT_RESCAN.
			 */
			recompute_limits(node);

			/* FALL THRU */

		case LIMIT_RESCAN:

			/*
			 * If backwards scan, just return NULL without changing state.
			 */
			if (!ScanDirectionIsForward(direction))
				return NULL;

			/*
			 * Check for empty window; if so, treat like empty subplan.
			 */
			if (node->count <= 0 && !node->noCount)
			{
				node->lstate = LIMIT_EMPTY;
				return NULL;
			}

			/*
			 * Fetch rows from subplan until we reach position > offset.
			 */
			for (;;)
			{
				slot = ExecProcNode(outerPlan);
				if (TupIsNull(slot))