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('','Hr');
insert into hy_dept(deptno,dname) values('','Dev');
insert into hy_dept(deptno,dname) values('','Qa');
insert into hy_dept(deptno,dname) values('','Sales');
insert into hy_dept(deptno,dname) values('','Mng');

insert into hy_emp(empno,ename,deptno,sal) values('','Andy','',1000);
insert into hy_emp(empno,ename,deptno,sal) values('','Bill','',2000);
insert into hy_emp(empno,ename,deptno,sal) values('','Cindy','',3000);
insert into hy_emp(empno,ename,deptno,sal) values('','Douglas','',4000);
insert into hy_emp(empno,ename,deptno,sal) values('','Edinburg','',5000);
insert into hy_emp(empno,ename,deptno,sal) values('','Felix','',6000);
insert into hy_emp(empno,ename,deptno,sal) values('','Hellen','',7000);
insert into hy_emp(empno,ename,deptno,sal) values('','Isis','',8000);
insert into hy_emp(empno,ename,deptno,sal) values('','Jean','',9000);
insert into hy_emp(empno,ename,deptno,sal) values('','King','',10000);
insert into hy_emp(empno,ename,deptno,sal) values('','Mac','',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。