oracle hint_leading
oracle hint_leading_merge
1,leading
1,比ordered功能更多
2,如下明确看出leading可以发变首表
3,如果指定了ordered,优先级高于leading
SQL> explain plan for select emp.empno,dept.dname from emp,dept where emp.deptno
=dept.deptno;
1,leading
1,比ordered功能更多
2,如下明确看出leading可以发变首表
3,如果指定了ordered,优先级高于leading
SQL> explain plan for select emp.empno,dept.dname from emp,dept where emp.deptno
=dept.deptno;
已解释。
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
Plan hash value: 615168685
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 240 | 5 (20)| 00:00:01 |
|* 1 | HASH JOIN | | 12 | 240 | 5 (20)| 00:00:01 |
| 2 | TABLE ACCESS FULL| DEPT | 8 | 104 | 2 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| EMP | 12 | 84 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 240 | 5 (20)| 00:00:01 |
|* 1 | HASH JOIN | | 12 | 240 | 5 (20)| 00:00:01 |
| 2 | TABLE ACCESS FULL| DEPT | 8 | 104 | 2 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| EMP | 12 | 84 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
---------------------------------------------------
1 - access("EMP"."DEPTNO"="DEPT"."DEPTNO")
已选择15行。
SQL> explain plan for select /*+ leading(emp dept) */ emp.empno,dept.dname from
emp,dept where emp.deptno=dept.deptno;
emp,dept where emp.deptno=dept.deptno;
已解释。
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
Plan hash value: 1123238657
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 240 | 5 (20)| 00:00:01 |
|* 1 | HASH JOIN | | 12 | 240 | 5 (20)| 00:00:01 |
| 2 | TABLE ACCESS FULL| EMP | 12 | 84 | 2 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| DEPT | 8 | 104 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 12 | 240 | 5 (20)| 00:00:01 |
|* 1 | HASH JOIN | | 12 | 240 | 5 (20)| 00:00:01 |
| 2 | TABLE ACCESS FULL| EMP | 12 | 84 | 2 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| DEPT | 8 | 104 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
---------------------------------------------------
1 - access("EMP"."DEPTNO"="DEPT"."DEPTNO")
已选择15行。
SQL>
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/9240380/viewspace-751379/ ，如需转载，请注明出处，否则将追究法律责任。
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