Function min_join_parameterization

src/backend/optimizer/util/relnode.c:1367–1396

* min_join_parameterization * * Determine the minimum possible parameterization of a joinrel, that is, the * set of other rels it contains LATERAL references to. We save this value in * the join's RelOptInfo. This function is split out of build_join_rel() * because join_is_legal() needs the value to check a prospective join. */

 * because join_is_legal() needs the value to check a prospective join.
 */
Relids
min_join_parameterization(PlannerInfo *root,
						  Relids joinrelids,
						  RelOptInfo *outer_rel,
						  RelOptInfo *inner_rel)
{
	Relids		result;

	/*
	 * Basically we just need the union of the inputs' lateral_relids, less
	 * whatever is already in the join.
	 *
	 * It's not immediately obvious that this is a valid way to compute the
	 * result, because it might seem that we're ignoring possible lateral refs
	 * of PlaceHolderVars that are due to be computed at the join but not in
	 * either input.  However, because create_lateral_join_info() already
	 * charged all such PHV refs to each member baserel of the join, they'll
	 * be accounted for already in the inputs' lateral_relids.  Likewise, we
	 * do not need to worry about doing transitive closure here, because that
	 * was already accounted for in the original baserel lateral_relids.
	 */
	result = bms_union(outer_rel->lateral_relids, inner_rel->lateral_relids);
	result = bms_del_members(result, joinrelids);

	/* Maintain invariant that result is exactly NULL if empty */
	if (bms_is_empty(result))
		result = NULL;

	return result;
}

/*
 * build_joinrel_tlist