Function ExecSetTupleBound

src/backend/executor/execProcnode.c:1419–1571

* ExecSetTupleBound * * Set a tuple bound for a planstate node. This lets child plan nodes * optimize based on the knowledge that the maximum number of tuples that * their parent will demand is limited. The tuple bound for a node may * only be changed between scans (i.e., after node initialization or just * before an ExecReScan call). * * Any negative tuples_needed value means "no limit"

 * only unchanging conditions are tested here.
 */
void
ExecSetTupleBound(int64 tuples_needed, PlanState *child_node)
{
	/*
	 * Since this function recurses, in principle we should check stack depth
	 * here.  In practice, it's probably pointless since the earlier node
	 * initialization tree traversal would surely have consumed more stack.
	 */

	if (IsA(child_node, SortState))
	{
		/*
		 * If it is a Sort node, notify it that it can use bounded sort.
		 *
		 * Note: it is the responsibility of nodeSort.c to react properly to
		 * changes of these parameters.  If we ever redesign this, it'd be a
		 * good idea to integrate this signaling with the parameter-change
		 * mechanism.
		 */
		SortState  *sortState = (SortState *) child_node;

		if (tuples_needed < 0)
		{
			/* make sure flag gets reset if needed upon rescan */
			sortState->bounded = false;
		}
		else
		{
			sortState->bounded = true;
			sortState->bound = tuples_needed;
		}
	}
	else if (IsA(child_node, IncrementalSortState))
	{
		/*
		 * If it is an IncrementalSort node, notify it that it can use bounded
		 * sort.
		 *
		 * Note: it is the responsibility of nodeIncrementalSort.c to react
		 * properly to changes of these parameters.  If we ever redesign this,
		 * it'd be a good idea to integrate this signaling with the
		 * parameter-change mechanism.
		 */
		IncrementalSortState *sortState = (IncrementalSortState *) child_node;

		if (tuples_needed < 0)
		{
			/* make sure flag gets reset if needed upon rescan */
			sortState->bounded = false;
		}
		else
		{
			sortState->bounded = true;
			sortState->bound = tuples_needed;
		}
	}
	else if (IsA(child_node, AppendState))
	{