Function add_partial_path_precheck

src/backend/optimizer/util/pathnode.c:963–1013

* add_partial_path_precheck * Check whether a proposed new partial path could possibly get accepted. * * Unlike add_path_precheck, we can ignore startup cost and parameterization, * since they don't matter for partial paths (see add_partial_path). But * we do want to make sure we don't add a partial path if there's already * a complete path that dominates it, since in that case the propos

 * is surely a loser.
 */
bool
add_partial_path_precheck(RelOptInfo *parent_rel, Cost total_cost,
						  List *pathkeys)
{
	ListCell   *p1;

	/*
	 * Our goal here is twofold.  First, we want to find out whether this path
	 * is clearly inferior to some existing partial path.  If so, we want to
	 * reject it immediately.  Second, we want to find out whether this path
	 * is clearly superior to some existing partial path -- at least, modulo
	 * final cost computations.  If so, we definitely want to consider it.
	 *
	 * Unlike add_path(), we always compare pathkeys here.  This is because we
	 * expect partial_pathlist to be very short, and getting a definitive
	 * answer at this stage avoids the need to call add_path_precheck.
	 */
	foreach(p1, parent_rel->partial_pathlist)
	{
		Path	   *old_path = (Path *) lfirst(p1);
		PathKeysComparison keyscmp;

		keyscmp = compare_pathkeys(pathkeys, old_path->pathkeys);
		if (keyscmp != PATHKEYS_DIFFERENT)
		{
			if (total_cost > old_path->total_cost * STD_FUZZ_FACTOR &&
				keyscmp != PATHKEYS_BETTER1)
				return false;
			if (old_path->total_cost > total_cost * STD_FUZZ_FACTOR &&
				keyscmp != PATHKEYS_BETTER2)
				return true;
		}
	}

	/*
	 * This path is neither clearly inferior to an existing partial path nor
	 * clearly good enough that it might replace one.  Compare it to
	 * non-parallel plans.  If it loses even before accounting for the cost of
	 * the Gather node, we should definitely reject it.
	 *
	 * Note that we pass the total_cost to add_path_precheck twice.  This is
	 * because it's never advantageous to consider the startup cost of a
	 * partial path; the resulting plans, if run in parallel, will be run to
	 * completion.
	 */
	if (!add_path_precheck(parent_rel, total_cost, total_cost, pathkeys,
						   NULL))
		return false;

	return true;
}


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