基于代价的优化器是很聪明的，在绝大多数情况下它会选择正确的优化器，减轻了DBA的负担。但有时它也聪明反被聪明误，选择了很差的执行计划，使某个语句的执行变得奇慢无比。此时就需要DBA进行人为的干预，告诉优化器使用我们指定的存取路径或连接类型生成执行计划，从 而使语句高效的运行。例如，如果我们认为对于一个特定的语句，执行全表扫描要比执行索引扫描更有效，则我们就可以指示优化器使用全表扫描。在Oracle 中，是通过为语句添加 Hints(提示)来实现干预优化器优化的目的。
不建议在代码中使用hint，在代码使用hint使得CBO无法根据实际的数据状态选择正确的执行计划。毕竟 数据是不断变化的， 10g以后的CBO也越来越完善，大多数情况下我们该让Oracle自行决定采用什么执行计划。Oracle Hints是一种机制，用来告诉优化器按照我们的告诉它的方式生成执行计划。我们可以用Oracle Hints来实现：
- Hints for Optimization Approaches and Goals
- Hints for Access Paths
- Hints for Query Transformations
- Hints for Join Orders
- Hints for Join Operations
- Hints for Parallel Execution
- Additional Hints
实现提示的语法：
{DELETE|INSERT|SELECT|UPDATE} /*+ hint [text] [hint[text]]... */
or
{DELETE|INSERT|SELECT|UPDATE} --+ hint [text] [hint[text]]...
Hints for Optimization Approaches and Goals
/*+ ALL_ROWS*/ 语句块选择基于成本的优化方法,并获得最佳吞吐量,使资源消耗最小化.
/*+ FIRST_ROWS(n)*/ 语句块选择基于成本的优化方法,并获得最佳响应时间,使资源消耗最小化.
/*+ CHOOSE*/ 语句块依赖统计信息来决定选择CBO还是RBO
/*+ RULE*/ 语句块选择基于规则的优化方法.
实例：
SQL> select /*+ALL_ROWS*/ * from emp,dept where emp.deptno=dept.deptno;
Execution Plan
----------------------------------------------------------
Plan hash value: 844388907
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 812 | 6(17)| 00:00:01 |
| 1 | MERGE JOIN | | 14 | 812 | 6(17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 80 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 14 | 532 | 4(25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | EMP | 14 | 532 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("EMP"."DEPTNO"="DEPT"."DEPTNO")
filter("EMP"."DEPTNO"="DEPT"."DEPTNO")
SQL> select /*+ FIRST_ROWS(1)*/ * from emp,dept where emp.deptno=dept.deptno;
14 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3625962092
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 58 | 3 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | | | | |
| 2 | NESTED LOOPS | | 1 | 58 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | EMP | 1 | 38 | 2 (0)| 00:00:01 |
|* 4 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 20 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("EMP"."DEPTNO"="DEPT"."DEPTNO")
Hints for Access Paths
/*+ FULL(TABLE)*/ 全表扫描
SQL> select empno from emp; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 179099197 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 4 | 1 (0)| 00:00:01 | | 1 | INDEX FULL SCAN | PK_EMP | 1 | 4 | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------
加hint后
SQL> select /*+ FULL(emp)*/ ename from emp; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 3956160932 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 6 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS FULL| EMP | 1 | 6 | 2 (0)| 00:00:01 | --------------------------------------------------------------------------
/*+ROWID(TABLE)*/
SQL> SELECT ROWID,EMPNO FROM EMP; ROWID EMPNO ------------------ ---------- AAASZHAAEAAAACXAAA 7369 AAASZHAAEAAAACXAAB 7499 AAASZHAAEAAAACXAAC 7521 AAASZHAAEAAAACXAAD 7566 AAASZHAAEAAAACXAAE 7654 AAASZHAAEAAAACXAAF 7698 AAASZHAAEAAAACXAAG 7782 AAASZHAAEAAAACXAAH 7788 AAASZHAAEAAAACXAAI 7839 AAASZHAAEAAAACXAAJ 7844 AAASZHAAEAAAACXAAK 7876 ROWID EMPNO ------------------ ---------- AAASZHAAEAAAACXAAL 7900 AAASZHAAEAAAACXAAM 7902 AAASZHAAEAAAACXAAN 7934 14 rows selected.
从上面的结果集中选取一个rowid，不加hint
SQL> SELECT * FROM EMP WHERE ROWID>=‘AAASZHAAEAAAACXAAA‘ AND EMPNO IN(7521,7654); Execution Plan ---------------------------------------------------------- Plan hash value: 2355049923 --------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 | | 1 | INLIST ITERATOR | | | | | | | 2 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 2 (0)| 00:00:01 | |* 3 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------------------
使用hint
SQL> SELECT /*+rowid(EMP)*/ * FROM EMP WHERE ROWID>=‘AAASZHAAEAAAACXAAA‘ AND EMPNO IN(7521,7654); Execution Plan ---------------------------------------------------------- Plan hash value: 2267975152 ------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 38 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS BY ROWID RANGE| EMP | 1 | 38 | 3 (0)| 00:00:01 | ------------------------------------------------------------------------------------
/*+ INDEX(TABLE INDEX_NAME) */ 对表选择索引的扫描方法. INDEX_NAME一定要大写
SQL> select /*+INDEX(emp PK_EMP)*/ * from emp; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 4170700152 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 2 (0)| 00:00:01 | | 2 | INDEX FULL SCAN | PK_EMP | 14 | | 1 (0)| 00:00:01 | --------------------------------------------------------------------------------------
/*+ INDEX_ASC(TABLE INDEX_NAME)*/ 表明对表选择索引升序的扫描方法. 建立索引时如果没有指定desc，那么INDEX_ASC和INDEX 提示表示相同意义。
SQL> select /*+INDEX_ASC(emp PK_EMP)*/ * from emp; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 4170700152 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 2 (0)| 00:00:01 | | 2 | INDEX FULL SCAN | PK_EMP | 14 | | 1 (0)| 00:00:01 | --------------------------------------------------------------------------------------
/*+ INDEX_DESC(TABLE INDEX_NAME)*/ 表明对表选择索引降序的扫描方法.
SQL> select /*+INDEX_DESC(emp PK_EMP)*/ * from emp; EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO ---------- ------------------------------ --------------------------- ---------- ------------------- ---------- ---------- ---------- 7934 MILLER CLERK 7782 1982/01/23 00:00:00 1300 10 7902 FORD ANALYST 7566 1981/12/03 00:00:00 3000 20 7900 JAMES CLERK 7698 1981/12/03 00:00:00 950 30 7876 ADAMS CLERK 7788 1987/05/23 00:00:00 1100 20 7844 TURNER SALESMAN 7698 1981/09/08 00:00:00 1500 0 30 7839 KING PRESIDENT 1981/11/17 00:00:00 5000 10 7788 SCOTT ANALYST 7566 1987/04/19 00:00:00 3000 20 7782 CLARK MANAGER 7839 1981/06/09 00:00:00 2450 10 7698 BLAKE MANAGER 7839 1981/05/01 00:00:00 2850 30 7654 MARTIN SALESMAN 7698 1981/09/28 00:00:00 1250 1400 30 7566 JONES MANAGER 7839 1981/04/02 00:00:00 2975 20 7521 WARD SALESMAN 7698 1981/02/22 00:00:00 1250 500 30 7499 ALLEN SALESMAN 7698 1981/02/20 00:00:00 1600 300 30 7369 SMITH CLERK 7902 1980/12/17 00:00:00 800 20 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 3088625055 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 2 (0)| 00:00:01 | | 2 | INDEX FULL SCAN DESCENDING| PK_EMP | 14 | | 1 (0)| 00:00:01 | --------------------------------------------------------------------------------------
上面的查询结果是按照empno降序排列的。
/*+INDEX_COMBINE(TABLE INDEX1 INDEX2 ...)*/
SQL> create bitmap index bidx_emp_sal on emp(sal); Index created. SQL> create bitmap index bidx_emp_hiredate on emp(hiredate); Index created.
SQL> SELECT * FROM EMP WHERE SAL<1500 AND HIREDATE<‘1981/06/09 00:00:00‘;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ------------------------------ --------------------------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980/12/17 00:00:00 800 20
7521 WARD SALESMAN 7698 1981/02/22 00:00:00 1250 500 30
Execution Plan
----------------------------------------------------------
Plan hash value: 1384570463
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 38 | 2 (0)| 00:00:01 |
| 2 | BITMAP CONVERSION TO ROWIDS| | | | | |
|* 3 | BITMAP INDEX RANGE SCAN | BIDX_EMP_HIREDATE | | | | |
--------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("SAL"<1500)
3 - access("HIREDATE"<TO_DATE(‘ 1981-06-09 00:00:00‘, ‘syyyy-mm-dd hh24:mi:ss‘))
filter("HIREDATE"<TO_DATE(‘ 1981-06-09 00:00:00‘, ‘syyyy-mm-dd hh24:mi:ss‘))
使用hint后
SQL> SELECT /*+INDEX_COMBINE( EMP BIDX_EMP_HIREDATE BIDX_EMP_SAL)*/ * FROM EMP WHERE SAL<1500 AND HIREDATE<‘1981/06/09 00:00:00‘;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ------------------------------ --------------------------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980/12/17 00:00:00 800 20
7521 WARD SALESMAN 7698 1981/02/22 00:00:00 1250 500 30
Execution Plan
----------------------------------------------------------
Plan hash value: 1332639593
--------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 38 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 38 | 2 (0)| 00:00:01 |
| 2 | BITMAP CONVERSION TO ROWIDS| | | | | |
| 3 | BITMAP AND | | | | | |
| 4 | BITMAP MERGE | | | | | |
|* 5 | BITMAP INDEX RANGE SCAN | BIDX_EMP_HIREDATE | | | | |
| 6 | BITMAP MERGE | | | | | |
|* 7 | BITMAP INDEX RANGE SCAN | BIDX_EMP_SAL | | | | |
--------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("HIREDATE"<TO_DATE(‘ 1981-06-09 00:00:00‘, ‘syyyy-mm-dd hh24:mi:ss‘))
filter("HIREDATE"<TO_DATE(‘ 1981-06-09 00:00:00‘, ‘syyyy-mm-dd hh24:mi:ss‘))
7 - access("SAL"<1500)
filter("SAL"<1500)
/*+ INDEX_JOIN(TABLE INDEX_NAME1 INDEX_NAME2) */
当谓词中引用的列都有索引的时候，可以通过指定采用索引关联的方式，来访问数据.选择列只能是索引中的列。
SQL> create index idx_emp_ename on emp(ename); Index created. SQL> select /*+ INDEX_JOIN( emp PK_EMP IDX_EMP_ENAME)*/ empno,ename from emp where ename=‘KING‘ and empno=7839; EMPNO ENAME ---------- ------------------------------ 7839 KING Execution Plan ---------------------------------------------------------- Plan hash value: 70197466 --------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 10 | 2 (50)| 00:00:01 | |* 1 | VIEW | index$_join$_001 | 1 | 10 | 2 (50)| 00:00:01 | |* 2 | HASH JOIN | | | | | | |* 3 | INDEX RANGE SCAN| PK_EMP | 1 | 10 | 0 (0)| 00:00:01 | |* 4 | INDEX RANGE SCAN| IDX_EMP_ENAME | 1 | 10 | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------------------
/*+ INDEX_FFS(TABLE INDEX_NAME) */对指定的表执行快速全索引扫描,而不是全表扫描的办法
SQL> select empno from emp; Execution Plan ---------------------------------------------------------- Plan hash value: 179099197 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 4 | 1 (0)| 00:00:01 | | 1 | INDEX FULL SCAN | PK_EMP | 1 | 4 | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------
我们加上hint后
SQL> select /*+INDEX_FFS(emp PK_EMP)*/ empno from emp order by empno; Execution Plan ---------------------------------------------------------- Plan hash value: 3618959410 -------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 14 | 56 | 3 (34)| 00:00:01 | | 1 | SORT ORDER BY | | 14 | 56 | 3 (34)| 00:00:01 | | 2 | INDEX FAST FULL SCAN| PK_EMP | 14 | 56 | 2 (0)| 00:00:01 | --------------------------------------------------------------------------------
/*+NO_INDEX(TABLE INDEX_NAME)*/ 不使用索引
SQL> select /*+NO_INDEX(emp PK_EMP)*/ empno from emp; Execution Plan ---------------------------------------------------------- Plan hash value: 3956160932 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 4 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS FULL| EMP | 1 | 4 | 2 (0)| 00:00:01 | --------------------------------------------------------------------------
/*+AND_EQUAL(TABLE INDEX1 INDEX2 ...)*/ index最少两个，最多不超过5个。
这个和INDEX_JOIN有点类似，但是INDEX_JOIN只能指定两个索引
SQL> CREATE INDEX IDX_EMP_JOB ON EMP(JOB); Index created. SQL> select /*+ AND_EQUAL(emp IDX_EMP_JOB IDX_EMP_ENAME)*/ empno,ename from emp where ENAME=‘KING‘ and JOB=‘SALESMAN‘; no rows selected Execution Plan ---------------------------------------------------------- Plan hash value: 1954919191 --------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 18 | 2 (0)| 00:00:01 | |* 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 18 | 2 (0)| 00:00:01 | | 2 | AND-EQUAL | | | | | | |* 3 | INDEX RANGE SCAN | IDX_EMP_ENAME | 1 | | 1 (0)| 00:00:01 | |* 4 | INDEX RANGE SCAN | IDX_EMP_JOB | 3 | | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------------------------
Hints for Query Transformations
/*+USE_CONCAT*/ 将WHERE 子句中的or或者in 查询转换成UNION ALL查询
SQL> SELECT /*+USE_CONCAT*/ * from emp where ename=‘KING‘ OR SAL>5000; Execution Plan ---------------------------------------------------------- Plan hash value: 1292243969 ----------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 4(0)| 00:00:01 | | 1 | CONCATENATION | | | | | | | 2 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 38 | 2(0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | IDX_EMP_ENAME | 1 | | 1(0)| 00:00:01 | |* 4 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 38 | 2(0)| 00:00:01 | | 5 | BITMAP CONVERSION TO ROWIDS| | | | | | |* 6 | BITMAP INDEX RANGE SCAN | BIDX_EMP_SAL | | | | | -----------------------------------------------------------------------------------------------
/*+NO_EXPAND*/ 与USE_CONCAT正好相反，就是阻止优化器将条件中带or或者in查询转换成UNION ALL
SQL> select * from emp where empno=7840 or ename=‘SCOTT‘; Execution Plan ---------------------------------------------------------- Plan hash value: 2037299637 ---------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 38 | 3 (0)| 00:00:01 | | 1 | CONCATENATION | | | | | | | 2 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 2 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | IDX_EMP_ENAME | 1 | | 1 (0)| 00:00:01 | |* 4 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 1 (0)| 00:00:01 | |* 5 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 0 (0)| 00:00:01 | ----------------------------------------------------------------------------------------------
加hint后
SQL> select /*+NO_EXPAND*/ * from emp where empno=7840 or ename=‘SCOTT‘; Execution Plan ---------------------------------------------------------- Plan hash value: 3956160932 -------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 2 | 76 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| EMP | 2 | 76 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------
/*+REWRITE（mview）*/ 使用物化视图重写sql
/*+NO_REWRITE*/ 不使用物化视图重写sql
/*+MERGE*/ 对视图查询进行合并。
看如下例子：
SQL> SELECT e1.ename, e1.sal, v.avg_sal FROM emp e1, (SELECT deptno, avg(sal) avg_sal FROM emp e2 GROUP BY deptno) v WHERE e1.deptno = v.deptno AND e1.sal > v.avg_sal; Execution Plan ---------------------------------------------------------- Plan hash value: 269884559 ----------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 29 | 8 (25)| 00:00:01 | |* 1 | HASH JOIN | | 1 | 29 | 8 (25)| 00:00:01 | | 2 | VIEW | | 3 | 48 | 4 (25)| 00:00:01 | | 3 | HASH GROUP BY | | 3 | 21 | 4 (25)| 00:00:01 | | 4 | TABLE ACCESS FULL| EMP | 14 | 98 | 3 (0)| 00:00:01 | | 5 | TABLE ACCESS FULL | EMP | 14 | 182 | 3 (0)| 00:00:01 | -----------------------------------------------------------------------------
先把v的结果集算出来，再和e1进行join运算。
如果使用hint呢。
SQL> SELECT /*+merge(v)*/e1.ename, e1.sal, v.avg_sal FROM emp e1, (SELECT deptno, avg(sal) avg_sal FROM emp e2 GROUP BY deptno) v WHERE e1.deptno = v.deptno AND e1.sal > v.avg_sal; Execution Plan ---------------------------------------------------------- Plan hash value: 2435006919 ----------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 4 | 128 | 8 (25)| 00:00:01 | |* 1 | FILTER | | | | | | | 2 | HASH GROUP BY | | 4 | 128 | 8 (25)| 00:00:01 | |* 3 | HASH JOIN | | 65 | 2080 | 7 (15)| 00:00:01 | | 4 | TABLE ACCESS FULL| EMP | 14 | 350 | 3 (0)| 00:00:01 | | 5 | TABLE ACCESS FULL| EMP | 14 | 98 | 3 (0)| 00:00:01 | -----------------------------------------------------------------------------
先将两表进行关联，再进行group by
/*NO_MERGE(VIEW)*/ 与MERGE操作正好相反。
Hints for Join Orders
/*+ORDERED*/ 根据表在FROM子句中的顺序,依次对其连接.
SQL> select * from emp e,dept d where e.deptno=d.deptno; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 844388907 ---------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 14 | 812 | 6 (17)| 00:00:01 | | 1 | MERGE JOIN | | 14 | 812 | 6 (17)| 00:00:01 | | 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 80 | 2 (0)| 00:00:01 | | 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 | |* 4 | SORT JOIN | | 14 | 532 | 4 (25)| 00:00:01 | | 5 | TABLE ACCESS FULL | EMP | 14 | 532 | 3 (0)| 00:00:01 | ----------------------------------------------------------------------------------------
虽然emp表写在前面，但是优化器并没有先处理emp表。
添加hint后
SQL> select /*+ORDERED*/ * from emp e,dept d where e.deptno=d.deptno; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 1123238657 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 14 | 812 | 7 (15)| 00:00:01 | |* 1 | HASH JOIN | | 14 | 812 | 7 (15)| 00:00:01 | | 2 | TABLE ACCESS FULL| EMP | 14 | 532 | 3 (0)| 00:00:01 | | 3 | TABLE ACCESS FULL| DEPT | 4 | 80 | 3 (0)| 00:00:01 | ---------------------------------------------------------------------------
Hints for Join Operations
/*+USE_NL(TABLE1 TABLE2)*/ 使用循环嵌套进行连接,并把指定的第一个表作为驱动表.
SQL> select /*+USE_NL(d e)*/ * from emp e,dept d where e.deptno=d.deptno; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 4192419542 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 14 | 812 | 10 (0)| 00:00:01 | | 1 | NESTED LOOPS | | 14 | 812 | 10 (0)| 00:00:01 | | 2 | TABLE ACCESS FULL| DEPT | 4 | 80 | 3 (0)| 00:00:01 | |* 3 | TABLE ACCESS FULL| EMP | 4 | 152 | 2 (0)| 00:00:01 | ---------------------------------------------------------------------------
/*+USE_MERGE(table1 table2)*/
SQL> alter session set optimizer_mode=first_rows_1; Session altered. SQL> select a.ename,b.ename from emp a,emp b where a.mgr=b.empno; 13 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 3355052392 --------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 20 | 3 (0)| 00:00:01 | | 1 | NESTED LOOPS | | | | | | | 2 | NESTED LOOPS | | 1 | 20 | 3 (0)| 00:00:01 | |* 3 | TABLE ACCESS FULL | EMP | 7 | 70 | 2 (0)| 00:00:01 | |* 4 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 0 (0)| 00:00:01 | | 5 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 10 | 1 (0)| 00:00:01 | ---------------------------------------------------------------------------------------
SQL> select /*+USE_MERGE(a b)*/ a.ename,b.ename from emp a,emp b where a.mgr=b.empno; 13 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 992080948 --------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 30 | 6 (17)| 00:00:01 | | 1 | MERGE JOIN | | 1 | 30 | 6 (17)| 00:00:01 | | 2 | TABLE ACCESS BY INDEX ROWID| EMP | 14 | 140 | 2 (0)| 00:00:01 | | 3 | INDEX FULL SCAN | PK_EMP | 14 | | 1 (0)| 00:00:01 | |* 4 | SORT JOIN | | 13 | 130 | 4 (25)| 00:00:01 | |* 5 | TABLE ACCESS FULL | EMP | 13 | 130 | 3 (0)| 00:00:01 | ---------------------------------------------------------------------------------------
/*+USE_HASH(table1 table2)*/ 将指定的表与其他表通过哈希连接方式连接起来.
SQL> select /*+USE_HASH(a b)*/ a.ename,b.ename from emp a,emp b where a.mgr=b.empno; 13 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 3638257876 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 20 | 7 (15)| 00:00:01 | |* 1 | HASH JOIN | | 1 | 20 | 7 (15)| 00:00:01 | | 2 | TABLE ACCESS FULL| EMP | 14 | 140 | 3 (0)| 00:00:01 | |* 3 | TABLE ACCESS FULL| EMP | 7 | 70 | 3 (0)| 00:00:01 | ---------------------------------------------------------------------------
/*+DRIVING_SITE(TABLE)*/ 此hint在使用dblink时有用。我们看如下例子
SQL> conn / as sysdba Connected. SQL> grant create database link to scott; Grant succeeded. SQL> conn scott/tiger Connected. SQL> create shared database link "db1" authenticated by SCOTT identified by "tiger" using ‘192.168.199.216:1521/11GDG1‘;
进行如下查询
SQL> select * from [email protected] e,dept d where e.deptno=d.deptno; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 2705760024 -------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT| -------------------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 14 | 812 | 6 (17)| 00:00:01 | | | | 1 | MERGE JOIN | | 14 | 812 | 6 (17)| 00:00:01 | | | | 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 80 | 2 (0)| 00:00:01 | | | | 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 | | | |* 4 | SORT JOIN | | 14 | 532 | 4 (25)| 00:00:01 | | | | 5 | REMOTE | EMP | 14 | 532 | 3 (0)| 00:00:01 | DB1 | R->S | --------------------------------------------------------------------------------------------------------
Oracle是将db1上的emp的数据传到本地，然后排序合并。如果emp的数据量非常大时，这样无疑是非常耗时的。如果我们可以将dept传给远端，在远端执行，结果返回到本地，那么执行的速度会比较快。
SQL> select /*+DRIVING_SITE(e)*/* from [email protected] e,dept d where e.deptno=d.deptno; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 2412741621 ----------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Inst |IN-OUT| ----------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT REMOTE| | 14 | 812 | 7 (15)| 00:00:01 | | | |* 1 | HASH JOIN | | 14 | 812 | 7 (15)| 00:00:01 | | | | 2 | REMOTE | DEPT | 4 | 80 | 3 (0)| 00:00:01 | ! | R->S | | 3 | TABLE ACCESS FULL | EMP | 14 | 532 | 3 (0)| 00:00:01 | DGTST | | -----------------------------------------------------------------------------------------------
/*+LEADING(TABLE)*/ 将指定的表作为连接次序中的首表.
SQL> select /*+LEADING(e)*/* from emp e,dept d where e.deptno=d.deptno; 14 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 1123238657 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 14 | 812 | 7 (15)| 00:00:01 | |* 1 | HASH JOIN | | 14 | 812 | 7 (15)| 00:00:01 | | 2 | TABLE ACCESS FULL| EMP | 14 | 532 | 3 (0)| 00:00:01 | | 3 | TABLE ACCESS FULL| DEPT | 4 | 80 | 3 (0)| 00:00:01 | ---------------------------------------------------------------------------
/*+HASH_AJ*/ , /*+MERGE_AJ*/, and /*+NL_AJ*/ 将not in 改写成反连接。 AJ = anti-join
SQL> select * from emp where empno not in (select /*+NL_AJ*/ mgr from emp where mgr is not null) ; 8 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 3509159946 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 8 | 336 | 24 (0)| 00:00:01 | | 1 | NESTED LOOPS ANTI | | 8 | 336 | 24 (0)| 00:00:01 | | 2 | TABLE ACCESS FULL| EMP | 14 | 532 | 3 (0)| 00:00:01 | |* 3 | TABLE ACCESS FULL| EMP | 6 | 24 | 2 (0)| 00:00:01 | ---------------------------------------------------------------------------
/*+HASH_SJ*/, /*+MERGE_SJ*/, and /*+NL_SJ*/ 将exists子句改写成半连接 SJ = semi-join
(一对多，只要有一个record 就 join成功)
SQL> select * from dept where exists (select * from emp where deptno=dept.deptno and sal<1000); Execution Plan ---------------------------------------------------------- Plan hash value: 1946750470 ------------------------------------------------------------------------------------------------ | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ------------------------------------------------------------------------------------------------ | 0 | SELECT STATEMENT | | 1 | 27 | 4 (25)| 00:00:01 | | 1 | NESTED LOOPS | | | | | | | 2 | NESTED LOOPS | | 1 | 27 | 4 (25)| 00:00:01 | | 3 | SORT UNIQUE | | 1 | 7 | 2 (0)| 00:00:01 | | 4 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 7 | 2 (0)| 00:00:01 | | 5 | BITMAP CONVERSION TO ROWIDS| | | | | | |* 6 | BITMAP INDEX RANGE SCAN | BIDX_EMP_SAL | | | | | |* 7 | INDEX UNIQUE SCAN | PK_DEPT | 1 | | 0 (0)| 00:00:01 | | 8 | TABLE ACCESS BY INDEX ROWID | DEPT | 1 | 20 | 1 (0)| 00:00:01 | ------------------------------------------------------------------------------------------------
添加hint
SQL> select * from dept where exists (select /*+HASH_SJ*/* from emp where deptno=dept.deptno and sal<1000); Execution Plan ---------------------------------------------------------- Plan hash value: 944460660 ---------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 2 | 54 | 6 (17)| 00:00:01 | |* 1 | HASH JOIN SEMI | | 2 | 54 | 6 (17)| 00:00:01 | | 2 | TABLE ACCESS FULL | DEPT | 4 | 80 | 3 (0)| 00:00:01 | | 3 | TABLE ACCESS BY INDEX ROWID | EMP | 1 | 7 | 2 (0)| 00:00:01 | | 4 | BITMAP CONVERSION TO ROWIDS| | | | | | |* 5 | BITMAP INDEX RANGE SCAN | BIDX_EMP_SAL | | | | | ----------------------------------------------------------------------------------------------
其他常用的hint
/*+ parallel(table_name n) */
在sql中指定执行的并行度，这个值将会覆盖自身的并行度
select /*+ parallel(t 4) */ count(*) from emp t;
/*+ no_parallel(table_name) */
在sql中指定执行的不使用并行
select /*+ no_parallel(t) */ count(*) from emp t;
/*+ append */以直接加载的方式将数据加载入库
insert into t /*+ append */ select * from t;
/*+ dynamic_sampling(table_name n) */
设置sql执行时动态采用的级别，这个级别为0~10
select /*+ dynamic_sampling(t 4) */ * from t where id > 1234
/*+ cache(table_name) */
进行全表扫描时将table置于LRU列表的最活跃端，类似于table的cache属性
select /*+ full(employees) cache(employees) */ last_name from employees
时间： 2024-10-13 14:19:07