Function add_partial_path

src/backend/optimizer/util/pathnode.c:845–951

* add_partial_path * Like add_path, our goal here is to consider whether a path is worthy * of being kept around, but the considerations here are a bit different. * A partial path is one which can be executed in any number of workers in * parallel such that each worker will generate a subset of the path's * overall result. * * As in add_path, the partial_pathlist is kept sorted

 *	  referenced by partial BitmapHeapPaths.
 */
void
add_partial_path(RelOptInfo *parent_rel, Path *new_path)
{
	bool		accept_new = true;	/* unless we find a superior old path */
	int			insert_at = 0;	/* where to insert new item */
	ListCell   *p1;

	/* Check for query cancel. */
	CHECK_FOR_INTERRUPTS();

	/* Path to be added must be parallel safe. */
	Assert(new_path->parallel_safe);

	/* Relation should be OK for parallelism, too. */
	Assert(parent_rel->consider_parallel);

	/*
	 * As in add_path, throw out any paths which are dominated by the new
	 * path, but throw out the new path if some existing path dominates it.
	 */
	foreach(p1, parent_rel->partial_pathlist)
	{
		Path	   *old_path = (Path *) lfirst(p1);
		bool		remove_old = false; /* unless new proves superior */
		PathKeysComparison keyscmp;

		/* Compare pathkeys. */
		keyscmp = compare_pathkeys(new_path->pathkeys, old_path->pathkeys);

		/* Unless pathkeys are incompatible, keep just one of the two paths. */
		if (keyscmp != PATHKEYS_DIFFERENT)
		{
			if (new_path->total_cost > old_path->total_cost * STD_FUZZ_FACTOR)
			{
				/* New path costs more; keep it only if pathkeys are better. */
				if (keyscmp != PATHKEYS_BETTER1)
					accept_new = false;
			}
			else if (old_path->total_cost > new_path->total_cost
					 * STD_FUZZ_FACTOR)
			{
				/* Old path costs more; keep it only if pathkeys are better. */
				if (keyscmp != PATHKEYS_BETTER2)
					remove_old = true;
			}
			else if (keyscmp == PATHKEYS_BETTER1)
			{
				/* Costs are about the same, new path has better pathkeys. */
				remove_old = true;
			}
			else if (keyscmp == PATHKEYS_BETTER2)
			{
				/* Costs are about the same, old path has better pathkeys. */
				accept_new = false;
			}
			else if (old_path->total_cost > new_path->total_cost * 1.0000000001)
			{
				/* Pathkeys are the same, and the old path costs more. */