Function get_singleton_append_subpath

src/backend/optimizer/path/allpaths.c:2387–2408

* get_singleton_append_subpath * Returns the single subpath of an Append/MergeAppend, or just * return 'path' if it's not a single sub-path Append/MergeAppend. * * Note: 'path' must not be a parallel-aware path. */

 * Note: 'path' must not be a parallel-aware path.
 */
static Path *
get_singleton_append_subpath(Path *path)
{
	Assert(!path->parallel_aware);

	if (IsA(path, AppendPath))
	{
		AppendPath *apath = (AppendPath *) path;

		if (list_length(apath->subpaths) == 1)
			return (Path *) linitial(apath->subpaths);
	}
	else if (IsA(path, MergeAppendPath))
	{
		MergeAppendPath *mpath = (MergeAppendPath *) path;

		if (list_length(mpath->subpaths) == 1)
			return (Path *) linitial(mpath->subpaths);
	}

	return path;
}

/*
 * set_dummy_rel_pathlist