Function create_gather_merge_path

src/backend/optimizer/util/pathnode.c:2911–2964

* create_gather_merge_path * * Creates a path corresponding to a gather merge scan, returning * the pathnode. */

 *	  the pathnode.
 */
GatherMergePath *
create_gather_merge_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
						 PathTarget *target, List *pathkeys,
						 Relids required_outer, double *rows)
{
	Assert(false);
	GatherMergePath *pathnode = makeNode(GatherMergePath);
	Cost		input_startup_cost = 0;
	Cost		input_total_cost = 0;

	Assert(subpath->parallel_safe);
	Assert(pathkeys);

	pathnode->path.pathtype = T_GatherMerge;
	pathnode->path.parent = rel;
	pathnode->path.param_info = get_baserel_parampathinfo(root, rel,
														  required_outer);
	pathnode->path.parallel_aware = false;

	pathnode->subpath = subpath;
	pathnode->num_workers = subpath->parallel_workers;
	pathnode->path.pathkeys = pathkeys;
	pathnode->path.pathtarget = target ? target : rel->reltarget;
	pathnode->path.rows += subpath->rows;

	if (pathkeys_contained_in(pathkeys, subpath->pathkeys))
	{
		/* Subpath is adequately ordered, we won't need to sort it */
		input_startup_cost += subpath->startup_cost;
		input_total_cost += subpath->total_cost;
	}
	else
	{
		/* We'll need to insert a Sort node, so include cost for that */
		Path		sort_path;	/* dummy for result of cost_sort */

		cost_sort(&sort_path,
				  root,
				  pathkeys,
				  subpath->total_cost,
				  subpath->rows,
				  subpath->pathtarget->width,
				  0.0,
				  work_mem,
				  -1);
		input_startup_cost += sort_path.startup_cost;
		input_total_cost += sort_path.total_cost;
	}

	cost_gather_merge(pathnode, root, rel, pathnode->path.param_info,
					  input_startup_cost, input_total_cost, rows);

	return pathnode;
}

/*
 * translate_sub_tlist - get subquery column numbers represented by tlist