Mysql创建外键约束会自动创建一个索引，但是Oracle不会这样做。
所以Oracle在创建外键之后，一定要手动创建索引。
如果没有在外键上创建索引，会经常遇到如下问题。
1.死锁


--创建实验表

create table emp as select * from hr.employees;

create table dept as select * from hr.departments;

alter table emp add constraint pk_emp primary key (employee_id);

alter table dept add constraint pk_dept primary key (department_id);

alter table emp add constraint fk_emp_dept foreign key (department_id) references dept(department_id) on delete cascade;

    如果删除任意一条Emp表的记录，都会导致dept表的所有记录不能删除。

SessionA，删除任意一条Emp表记录，不提交


另一个SessionB,他删除任何dept表的记录，都会被阻塞。

当然，对Dept表的插入和更新不受影响
    (假设不更新主键字段)

2.性能下降
查询某个部门的员工信息。

select employee_id,dept.department_name,first_name

from dept

inner join emp

on emp.department_id=dept.department_id

where emp.department_id=80;

如果没有索引,则emp表全表扫描

如果外键建立索引，则优化的效果显而易见


下面是Tom给出的脚本，检查外键无索引的表

COLUMN COLUMNS format a30 word_wrapped

COLUMN tablename format a15 word_wrapped

COLUMN constraint_name format a15 word_wrapped

SELECT TABLE_NAME,

       CONSTRAINT_NAME,

       CNAME1 || NVL2(CNAME2, ',' || CNAME2, NULL) ||

       NVL2(CNAME3, ',' || CNAME3, NULL) ||

       NVL2(CNAME4, ',' || CNAME4, NULL) ||

       NVL2(CNAME5, ',' || CNAME5, NULL) ||

       NVL2(CNAME6, ',' || CNAME6, NULL) ||

       NVL2(CNAME7, ',' || CNAME7, NULL) ||

       NVL2(CNAME8, ',' || CNAME8, NULL) COLUMNS

  FROM (SELECT B.TABLE_NAME,

               B.CONSTRAINT_NAME,

               MAX(DECODE(POSITION, 1, COLUMN_NAME, NULL)) CNAME1,

               MAX(DECODE(POSITION, 2, COLUMN_NAME, NULL)) CNAME2,

               MAX(DECODE(POSITION, 3, COLUMN_NAME, NULL)) CNAME3,

               MAX(DECODE(POSITION, 4, COLUMN_NAME, NULL)) CNAME4,

               MAX(DECODE(POSITION, 5, COLUMN_NAME, NULL)) CNAME5,

               MAX(DECODE(POSITION, 6, COLUMN_NAME, NULL)) CNAME6,

               MAX(DECODE(POSITION, 7, COLUMN_NAME, NULL)) CNAME7,

               MAX(DECODE(POSITION, 8, COLUMN_NAME, NULL)) CNAME8,

               COUNT(*) COL_CNT

          FROM (SELECT SUBSTR(TABLE_NAME, 1, 30) TABLE_NAME,

                       SUBSTR(CONSTRAINT_NAME, 1, 30) CONSTRAINT_NAME,

                       SUBSTR(COLUMN_NAME, 1, 30) COLUMN_NAME,

                       POSITION

                  FROM USER_CONS_COLUMNS) A,

               USER_CONSTRAINTS B

         WHERE A.CONSTRAINT_NAME = B.CONSTRAINT_NAME

           AND B.CONSTRAINT_TYPE = 'R'

         GROUP BY B.TABLE_NAME, B.CONSTRAINT_NAME) CONS

WHERE COL_CNT > ALL

(SELECT COUNT(*)

          FROM USER_IND_COLUMNS I

         WHERE I.TABLE_NAME = CONS.TABLE_NAME

           AND I.COLUMN_NAME IN (CNAME1, CNAME2, CNAME3, CNAME4, CNAME5,

                CNAME6, CNAME7, CNAME8)

           AND I.COLUMN_POSITION <= CONS.COL_CNT

         GROUP BY I.INDEX_NAME)

/

参考：
http://blog.csdn.net/dba_waterbin/article/details/9045483

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外键一定要加索引

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