Function create_merge_append_path

src/backend/optimizer/util/pathnode.c:1578–1677

* create_merge_append_path * Creates a path corresponding to a MergeAppend plan, returning the * pathnode. */

 *	  pathnode.
 */
MergeAppendPath *
create_merge_append_path(PlannerInfo *root,
						 RelOptInfo *rel,
						 List *subpaths,
						 List *pathkeys,
						 Relids required_outer)
{
	MergeAppendPath *pathnode = makeNode(MergeAppendPath);
	Cost		input_startup_cost;
	Cost		input_total_cost;
	ListCell   *l;

	pathnode->path.pathtype = T_MergeAppend;
	pathnode->path.parent = rel;
	pathnode->path.pathtarget = rel->reltarget;
	pathnode->path.param_info = get_appendrel_parampathinfo(rel,
															required_outer);
	pathnode->path.parallel_aware = false;
	pathnode->path.parallel_safe = rel->consider_parallel;
	pathnode->path.parallel_workers = 0;
	pathnode->path.pathkeys = pathkeys;
	pathnode->subpaths = subpaths;

	/*
	 * Apply query-wide LIMIT if known and path is for sole base relation.
	 * (Handling this at this low level is a bit klugy.)
	 */
	if (bms_equal(rel->relids, root->all_baserels))
		pathnode->limit_tuples = root->limit_tuples;
	else
		pathnode->limit_tuples = -1.0;

	/*
	 * Add Motions to the child nodes as needed, and determine the locus
	 * of the MergeAppend itself.
	 */
	if (!set_append_path_locus(root, (Path *) pathnode, rel, pathkeys, 0, false))
		return NULL;

	/*
	 * Add up the sizes and costs of the input paths.
	 */
	pathnode->path.rows = 0;
	input_startup_cost = 0;
	input_total_cost = 0;
	foreach(l, subpaths)
	{
		Path	   *subpath = (Path *) lfirst(l);

		pathnode->path.rows += subpath->rows;
		pathnode->path.parallel_safe = pathnode->path.parallel_safe &&
			subpath->parallel_safe;

		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;