两表关联查询-创建约束，外键对比

Posted 2020-10-12

--开启执行计划：

set autotrace on

SP2-0618: Cannot find the Session Identifier. Check PLUSTRACE role is enabled

找不到会话标识符，启用检查
SP2-0611: Error enabling STATISTICS report

错误：统计报告

grant plustrace to scott
*
ERROR at line 1:
ORA-01919: role ‘PLUSTRACE‘ does not exist                ---角色不存在

SQL> select * from dba_roles where role=‘PLUSTRACE‘; ---查询角色

no rows selected

SQL> @?/sqlplus/admin/plustrce.sql                              -----执行脚本
SQL> drop role plustrace;
drop role plustrace
*
ERROR at line 1:
ORA-01919: role ‘PLUSTRACE‘ does not exist

SQL> create role plustrace;

Role created.

SQL> grant select on v_$sesstat to plustrace;

Grant succeeded.

SQL> grant select on v_$statname to plustrace;

Grant succeeded.

SQL> grant select on v_$mystat to plustrace;

Grant succeeded.

SQL> grant plustrace to dba with admin option;

Grant succeeded.

SQL> set echo off

-----------------------------

grant plustrace to scott;

SQL> select USERNAME,GRANTED_ROLE from user_role_privs;

USERNAME GRANTED_ROLE
------------------------------ ------------------------------
SCOTT CONNECT
SCOTT PLUSTRACE
SCOTT RESOURCE

----这是最后的：类似的查询；

select empno,ename,sal from scott.emp e,scott.dept d where e.deptno=d.deptno; 

--生成测试：

SQL> create table emp1 as select * from emp;

SQL> create table dept2 as select * from dept;

**********测试查询一（两个表，没有索引，没有约束）*********************************

SQL> set autotrace traceonly;
SQL> select empno,ename,sal from emp1 e,dept2 d where e.deptno=d.deptno;

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 826 | 6 (0)| 00:00:01 |
|* 1 | HASH JOIN | | 14 | 826 | 6 (0)| 00:00:01 |
| 2 | TABLE ACCESS FULL| DEPT2 | 4 | 52 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| EMP1 | 14 | 644 | 3 (0)| 00:00:01 |

1 - access("E"."DEPTNO"="D"."DEPTNO")

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

          7  consistent gets

exec dbms_stats.gather_table_stats(ownname=>‘SCOTT‘,tabname=>‘EMP1‘);

*****************************************测试查询二（dept2表中建立了主键约束（建立了索引））*******************************************

--创建外键约束：

alter table emp1 add constraint emp_forkey foreign key(deptno) references dept2(deptno)
*
ERROR at line 1:
ORA-02270: no matching unique or primary key for this column-list

                --没有匹配的这个列表唯一或主键

子表创建与父表的：外键约束，必须要明确父表关联列，有主键或者唯一约束；

SQL> alter table dept2 add constraint p_k primary key(deptno);

SQL> select owner,constraint_name from user_constraints ;

OWNER CONSTRAINT_NAME
-------------------- --------------------
SCOTT P_K

SQL> select INDEX_NAME,TABLE_NAME from user_indexes;

INDEX_NAME TABLE_NAME
------------------------------ ------------------------------
P_K DEPT2

--建立主键约束后，父表dept2，的列deptno列自动生成索引；

SQL> set autotrac traceonly
SQL> select empno,ename,sal from emp1 e,dept2 d where e.deptno=d.deptno;

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |        *成本从6减为3
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 420 | 3 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 14 | 420 | 3 (0)| 00:00:01 |             *********从哈希转为嵌套循环
| 2 | TABLE ACCESS FULL| EMP1 | 14 | 238 | 3 (0)| 00:00:01 |  
|* 3 | INDEX UNIQUE SCAN| P_K | 1 | 13 | 0 (0)| 00:00:01 |  *********全表扫描，转为索引范围

3 - access("E"."DEPTNO"="D"."DEPTNO")

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

          7  consistent gets

 

***************************************测试三（emp1与dept2建立主外键约束）**************************************

SQL> set autotrace off

SQL> alter table emp1 add constraint f_y foreign key(deptno) references dept2(deptno);

SQL> select owner,constraint_name from user_constraints;

OWNER CONSTRAINT_NAME
-------------------- --------------------
SCOTT P_K
SCOTT F_Y

SQL> select index_name,table_name from user_indexes where table_name=‘EMP1‘;

no rows selected

select empno,ename,sal from emp1 e,dept2 d where e.deptno=d.deptno;

| 0 | SELECT STATEMENT | | 14 | 238 | 3 (0)| 00:00:01 |   ---因为测试数据量少，成本没有变化
|* 1 | TABLE ACCESS FULL| EMP1 | 14 | 238 | 3 (0)| 00:00:01 |  --对DEPT2表根本没有查询；
----------

   1 - filter("E"."DEPTNO" IS NOT NULL)         ---直接根据dept2,deptno列非空(主键，非空且唯一）；

14 rows selected.

 

*******测试五、测试四的基础上插入一条数据*******

insert into emp1(empno,ename,deptno) values(8000,‘YANG‘,null);

select empno,ename,sal from emp1 e,dept2 d where e.deptno=d.deptno;

--因为查询是等值连接，所以，执行计划不受影响；

但是会存在脏数据：对于外键约束来说，子表中的数据必须在父表中有匹配值，本次测试中，插入了Null，值没有报错；

insert into emp1(empno,ename,deptno) values(8100,‘YG‘,11);

ERROR at line 1:
ORA-02291: integrity constraint (SCOTT.F_Y) violated - parent key not found  外键还是好的；

小结：主外键约束，要保证数据干净，子表中，关联列加上not null 约束靠谱；

alter table emp1 modify deptno not null
*
ERROR at line 1:
ORA-02296: cannot enable (SCOTT.) - null values found

SQL> delete emp1 where deptno is null;

SQL> alter table emp1 modify deptno not null；