code: public static FlinkJoinType getFlinkJoinType(Join join) {
  		if (join instanceof SemiJoin) {
  			// TODO supports ANTI
  			return FlinkJoinType.SEMI;
  		} else {
  			return toFlinkJoinType(join.getJoinType());
  		}
  	}
docs: Gets {@link FlinkJoinType} of the input Join RelNode.