莫名其妙的Explain Plan

两张表的建表语句：

CREATE TABLE hy_emp
(
empno NUMBER(8,0) not null primary key,
ename NVARCHAR2(60) not null,
deptno NUMBER(8,0) not null,
sal NUMBER(10,0) DEFAULT 0 not null 
)


CREATE TABLE hy_dept
(
deptno NUMBER(8,0) not null primary key,
dname NVARCHAR2(60) not null
)

插值语句：

insert into hy_dept(deptno,dname) values('1','Hr');
insert into hy_dept(deptno,dname) values('2','Dev');
insert into hy_dept(deptno,dname) values('3','Qa');
insert into hy_dept(deptno,dname) values('4','Sales');
insert into hy_dept(deptno,dname) values('5','Mng');

insert into hy_emp(empno,ename,deptno,sal) values('1','Andy','1',1000);
insert into hy_emp(empno,ename,deptno,sal) values('2','Bill','2',2000);
insert into hy_emp(empno,ename,deptno,sal) values('3','Cindy','3',3000);
insert into hy_emp(empno,ename,deptno,sal) values('4','Douglas','4',4000);
insert into hy_emp(empno,ename,deptno,sal) values('5','Edinburg','5',5000);
insert into hy_emp(empno,ename,deptno,sal) values('6','Felix','1',6000);
insert into hy_emp(empno,ename,deptno,sal) values('7','Hellen','2',7000);
insert into hy_emp(empno,ename,deptno,sal) values('8','Isis','3',8000);
insert into hy_emp(empno,ename,deptno,sal) values('9','Jean','4',9000);
insert into hy_emp(empno,ename,deptno,sal) values('10','King','5',10000);
insert into hy_emp(empno,ename,deptno,sal) values('11','Mac','1',11000);

单表的Explain plan还好，两张表的实在让人莫名其妙：

留待日后分辨吧。