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Full Hint

2014-12-31 11:52 721 查看

Full Hint可以提示优化器对指定的表进行全表扫描，需要注意的是Full一次只能对一张表起作用。

下面是实验过程

SQL> select * from v$version;

BANNER

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

Personal Oracle Database 10g Release 10.2.0.1.0 - Production

PL/SQL Release 10.2.0.1.0 - Production

CORE 10.2.0.1.0
Production

TNS for 32-bit Windows: Version 10.2.0.1.0 - Production

NLSRTL Version 10.2.0.1.0 - Production

这是我的oracle环境

SQL> desc test1;

名称是否为空? 类型

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

EMPNO NUMBER(4)

ENAME VARCHAR2(10)

JOB VARCHAR2(9)

MGR NUMBER(4)

HIREDATE DATE

SAL NUMBER(7,2)

COMM NUMBER(7,2)

DEPTNO NUMBER(2)

SQL> desc test2

名称是否为空? 类型

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

EMPNO NUMBER(4)

ENAME VARCHAR2(10)

JOB VARCHAR2(9)

MGR NUMBER(4)

HIREDATE DATE

SAL NUMBER(7,2)

COMM NUMBER(7,2)

DEPTNO NUMBER(2)

SQL> create index pk_test1 on test1(empno);

索引已创建。

SQL> create index pk_test2 on test2(empno);

索引已创建。

SQL> select test1.empno,test2.empno from test1,test2

2 where test1.empno=test2.empno;

已选择14行。

执行计划

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

Plan hash value: 1756776100



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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 14 | 364 | 1 (0)| 00:00:01 |

| 1 | NESTED LOOPS | | 14 | 364 | 1 (0)| 00:00:01 |

| 2 | INDEX FULL SCAN | PK_TEST1 | 14 | 182 | 1 (0)| 00:00:01 |

|* 3 | INDEX RANGE SCAN| PK_TEST2 | 1 | 13 | 0 (0)| 00:00:01 |

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



Predicate Information (identified by operation id):

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



3 - access("TEST1"."EMPNO"="TEST2"."EMPNO")



Note

-----

- dynamic sampling used for this statement
初始情况下，优化器选择对两张表进行索引扫描

SQL> select /*+full(test1)*/ test1.empno,test2.empno from test1,test2

2 where test1.empno=test2.empno;

已选择14行。

执行计划

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

Plan hash value: 3442032678



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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 14 | 364 | 3 (0)| 00:00:01 |

| 1 | NESTED LOOPS | | 14 | 364 | 3 (0)| 00:00:01 |

| 2 | TABLE ACCESS FULL| TEST1 | 14 | 182 | 3 (0)| 00:00:01 |

|* 3 | INDEX RANGE SCAN | PK_TEST2 | 1 | 13 | 0 (0)| 00:00:01 |

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



Predicate Information (identified by operation id):

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



3 - access("TEST1"."EMPNO"="TEST2"."EMPNO")



Note

-----

- dynamic sampling used for this statement
使用full干预后，对test1进行全表扫描

SQL> select /*+full(test1 test2)*/ test1.empno,test2.empno from test1,test2

2 where test1.empno=test2.empno;

已选择14行。

执行计划

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

Plan hash value: 1756776100



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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

------------------------------------------------
4000
------------------------------

| 0 | SELECT STATEMENT | | 14 | 364 | 1 (0)| 00:00:01 |

| 1 | NESTED LOOPS | | 14 | 364 | 1 (0)| 00:00:01 |

| 2 | INDEX FULL SCAN | PK_TEST1 | 14 | 182 | 1 (0)| 00:00:01 |

|* 3 | INDEX RANGE SCAN| PK_TEST2 | 1 | 13 | 0 (0)| 00:00:01 |

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



Predicate Information (identified by operation id):

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



3 - access("TEST1"."EMPNO"="TEST2"."EMPNO")



Note

-----

- dynamic sampling used for this statement
使用full(test1 test2)进行干预后，发现没有全表扫描，说明full一次只能对一张表起作用

SQL> select /*+full(test1) full(test2)*/ test1.empno,test2.empno from test1,test2

2 where test1.empno=test2.empno;

已选择14行。

执行计划

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

Plan hash value: 497311279



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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 14 | 364 | 7 (15)| 00:00:01 |

|* 1 | HASH JOIN | | 14 | 364 | 7 (15)| 00:00:01 |

| 2 | TABLE ACCESS FULL| TEST1 | 14 | 182 | 3 (0)| 00:00:01 |

| 3 | TABLE ACCESS FULL| TEST2 | 14 | 182 | 3 (0)| 00:00:01 |

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



Predicate Information (identified by operation id):

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



1 - access("TEST1"."EMPNO"="TEST2"."EMPNO")



Note

-----

- dynamic sampling used for this statement

使用/*+full(test1) full(test2)*/ 进行干预后，优化器对test1和test2都进行了全表扫描，进一步说明full一次只能对一张表起作用。

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