[Oracle] 谈谈主外键设计

主外键有两大特点：

1）主键本身是一个唯一索引，保证主键所在列的唯一性；

2）外键列指必须在主表中的主键列有相应记录。

外键上一定要建索引

我们知道，主键本身是一个唯一索引，外键是一个约束，默认情况下没有索引，但在实际使用中强烈建议在外键上建索引，下面看两个例子：

SQL> alter database mount;

Database altered.

SQL> alter database open;

Database altered.

SQL> create table p (id number, name varchar2(30));

Table created.

SQL> alter table p add constraint pk primary key(id);

Table altered.

SQL> create table f (id number, pid number, name varchar2(30));

Table created.

SQL> alter table f add constraint fk foreign key(pid) references p(id);

Table altered.

SQL> insert into p select rownum,table_name from dba_tables;

1206 rows created.

SQL> insert into f select rownum,mod(rownum,1000)+1,object_name from dba_objects;

14090 rows created.

SQL> commit;

Commit complete.

上面语句分别创建了两个表，其中表p有主键，另一个表f有外键。但我们没有在外键上创建索引，下面我们看下这两表关联的执行计划和性能：

SQL> set autotrace traceonly
SQL> set line 1000
SQL> select p.id,p.name,f.name from p,f where p.id=f.pid and p.id=880;

14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3936432439

-------------------------------------------------------------------------------------
| Id  | Operation                    | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |      |    14 |   840 |    19   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |      |    14 |   840 |    19   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| P    |     1 |    30 |     0   (0)| 00:00:01 |
|*  3 |    INDEX UNIQUE SCAN         | PK   |     1 |       |     0   (0)| 00:00:01 |
|*  4 |   TABLE ACCESS FULL          | F    |    14 |   420 |    19   (0)| 00:00:01 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("P"."ID"=880)
   4 - filter("F"."PID"=880)

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          5  recursive calls
          0  db block gets
        137  consistent gets
          0  physical reads
          0  redo size
       1094  bytes sent via SQL*Net to client
        524  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         14  rows processed

从执行计划中，我们可以看出，对f表进行了全表扫描，试想一下，如果p表不是返回1条记录，而是返回多条记录，那f表是不是要做多次的全部扫描？答案是肯定的，为了避免对f表的全表扫描，我们应该在外键上创建索引，如下所示：

SQL> create index fk on f(pid);

Index created.

SQL> set autotrace traceonly
SQL> set line 1000
SQL> select p.id,p.name,f.name from p,f where p.id=f.pid and p.id=880;

14 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2077701003

-------------------------------------------------------------------------------------
| Id  | Operation                    | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |      |    14 |   840 |    16   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |      |    14 |   840 |    16   (0)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID| P    |     1 |    30 |     0   (0)| 00:00:01 |
|*  3 |    INDEX UNIQUE SCAN         | PK   |     1 |       |     0   (0)| 00:00:01 |
|   4 |   TABLE ACCESS BY IN