Function add_path_precheck

src/backend/optimizer/util/pathnode.c:740–804

* add_path_precheck * Check whether a proposed new path could possibly get accepted. * We assume we know the path's pathkeys and parameterization accurately, * and have lower bounds for its costs. * * Note that we do not know the path's rowcount, since getting an estimate for * that is too expensive to do before prechecking. We assume here that paths * of a superset parameterization

 * so the required information has to be passed piecemeal.
 */
bool
add_path_precheck(RelOptInfo *parent_rel,
				  Cost startup_cost, Cost total_cost,
				  List *pathkeys, Relids required_outer)
{
	List	   *new_path_pathkeys;
	bool		consider_startup;
	ListCell   *p1;

	/* Pretend parameterized paths have no pathkeys, per add_path policy */
	new_path_pathkeys = required_outer ? NIL : pathkeys;

	/* Decide whether new path's startup cost is interesting */
	consider_startup = required_outer ? parent_rel->consider_param_startup : parent_rel->consider_startup;

	foreach(p1, parent_rel->pathlist)
	{
		Path	   *old_path = (Path *) lfirst(p1);
		PathKeysComparison keyscmp;

		/*
		 * We are looking for an old_path with the same parameterization (and
		 * by assumption the same rowcount) that dominates the new path on
		 * pathkeys as well as both cost metrics.  If we find one, we can
		 * reject the new path.
		 *
		 * Cost comparisons here should match compare_path_costs_fuzzily.
		 */
		if (total_cost > old_path->total_cost * STD_FUZZ_FACTOR)
		{
			/* new path can win on startup cost only if consider_startup */
			if (startup_cost > old_path->startup_cost * STD_FUZZ_FACTOR ||
				!consider_startup)
			{
				/* new path loses on cost, so check pathkeys... */
				List	   *old_path_pathkeys;

				old_path_pathkeys = old_path->param_info ? NIL : old_path->pathkeys;
				keyscmp = compare_pathkeys(new_path_pathkeys,
										   old_path_pathkeys);
				if (keyscmp == PATHKEYS_EQUAL ||
					keyscmp == PATHKEYS_BETTER2)
				{
					/* new path does not win on pathkeys... */
					if (bms_equal(required_outer, PATH_REQ_OUTER(old_path)))
					{
						/* Found an old path that dominates the new one */
						return false;
					}
				}
			}
		}
		else
		{
			/*
			 * Since the pathlist is sorted by total_cost, we can stop looking
			 * once we reach a path with a total_cost larger than the new
			 * path's.