Oracle Hint 学习之一
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APPEDND hint :用于控制insert 语句是否能以直接路径插入的方式插入数据。
CACHE hint：用于控制目标sql在执行时是否将全表扫描目标表的数据块放到buffer cache的LRU链表的热端。
MONITER hint：用于控制被执行的目标sql是否被sql monitor监控
Gather_plan_statistics hint:用于在目标sql执行时收集一些额外的统计信息：
SQL> select t1.empno,t1.ename,t2.dname from emp t1,dept t2 where t1.deptno=t2.deptno;
EMPNO ENAME DNAME
---------- ---------- --------------
7782 CLARK ACCOUNTING
，，，，
14 rows selected.
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID4m81jub7yju91, child number 0
-------------------------------------
select t1.empno,t1.ename,t2.dname from
emp t1,dept t2 where t1.deptno=t2.deptno
Plan hash value: 844388907
-----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 14 |00:00:00.01 | 10 | | | |
| 1 | MERGE JOIN | | 1 | 14 | 14 |00:00:00.01 | 10 | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 4 |4 |00:00:00.01 | 4 | | | |
| 3 | INDEX FULL SCAN | PK_DEPT | 1 | 4 |4 |00:00:00.01 | 2 | | | |
|* 4 | SORT JOIN | | 4 | 14 | 14 |00:00:00.01 | 6 | 2048 | 2048 | 2048 (0)|
| 5 | TABLE ACCESS FULL | EMP | 1 | 14 | 14 |00:00:00.01 | 6 | | | |
-----------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T1"."DEPTNO"="T2"."DEPTNO")
filter("T1"."DEPTNO"="T2"."DEPTNO")
24 rows selected.不加hint，看不到上面starts类似的执行计划：
SQL> select t1.empno,t1.ename,t2.dname from emp t1,dept t2 where t1.deptno=t2.deptno;
EMPNO ENAME DNAME
---------- ---------- --------------
7782 CLARK ACCOUNTING
，，，，
14 rows selected.
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID4m81jub7yju91, child number 0
-------------------------------------
select t1.empno,t1.ename,t2.dname from
emp t1,dept t2 where t1.deptno=t2.deptno
Plan hash value: 844388907
-----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 14 |00:00:00.01 | 10 | | | |
| 1 | MERGE JOIN | | 1 | 14 | 14 |00:00:00.01 | 10 | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 4 |4 |00:00:00.01 | 4 | | | |
| 3 | INDEX FULL SCAN | PK_DEPT | 1 | 4 |4 |00:00:00.01 | 2 | | | |
|* 4 | SORT JOIN | | 4 | 14 | 14 |00:00:00.01 | 6 | 2048 | 2048 | 2048 (0)|
| 5 | TABLE ACCESS FULL | EMP | 1 | 14 | 14 |00:00:00.01 | 6 | | | |
-----------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T1"."DEPTNO"="T2"."DEPTNO")
filter("T1"."DEPTNO"="T2"."DEPTNO")
24 rows selected.SQL> select * from scott.emp where empno=7369; --错误的写法
SQL> select * from scott.emp where empno=7369; --正确的写法SQL> select * from scott.emp t1 where empno=7369; --HINT中指定别名，否则无效针对query block，hint生效范围仅限于它本身所在的。
SQL> select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO');SQL> select t1.ename,t1.deptno from t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO'); --该HINT对T2表不生效HINT中出现query block其格式必须是“@query block名称”。
方法一：
SQL> select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO');方法二：
SQL> select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO');方法三：（自定义qb_name）
SQL> select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO');SQL> select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO');SQL> set autot off;
SQL> select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO');
ENAME DEPTNO
---------- ----------
ALLEN 30
WARD 30
MARTIN 30
BLAKE 30
TURNER 30
JAMES 30
ALLEN 30
WARD 30
24 rows selected.
SQL> select * from table(dbms_xplan.display_cursor(null,null,'advanced'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID3v4x69w2mvqgs, child number 0
-------------------------------------
select t1.ename,t1.deptno from t1 where t1.deptno in (select t2.deptno
from dept t2 where t2.loc='CHICAGO')
Plan hash value: 2392421419
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 5 (100)| |
|* 1 | HASH JOIN | | 19 |380 | 5 (0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 11 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | IDX_DEPT_LOC | 1 | | 1 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | T1 | 56 |504 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$5DA710D3
2 - SEL$5DA710D3 / T2@SEL$2
3 - SEL$5DA710D3 / T2@SEL$2
4 - SEL$5DA710D3 / T1@SEL$1
Outline Data
-------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."DEPTNO"="T2"."DEPTNO")
3 - access("T2"."LOC"='CHICAGO')
Column Projection Information (identified by operation id):
-----------------------------------------------------------
1 - (#keys=1) "T1"."DEPTNO"[NUMBER,22], "T1"."ENAME"[VARCHAR2,10]
2 - "T2"."DEPTNO"[NUMBER,22]
3 - "T2".ROWID[ROWID,10]
4 - "T1"."ENAME"[VARCHAR2,10], "T1"."DEPTNO"[NUMBER,22]
59 rows selected.上述执行计划中：T2@SEL$2 和T1@SEL$1 query block ，而SEL$5DA710D3是一次查询转换（包含子查询展开，视图合并，连接谓词推入）而形成的新的query block。
Outline data，是用来固定执行计划的内部hint组合，非常全面的组合，比一般hint更加可靠：
在emp deptno建立索引，让sql走NL:
SQL> select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO');
6 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 902326130
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5 | 100 | 3 (0)| 00:00:01 |
| 1 | NESTED LOOPS | | 5 | 100 | 3 (0)| 00:00:01 |
| 2 | NESTED LOOPS | | 5 | 100 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 11 | 2 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IDX_DEPT_LOC | 1 | | 1 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | IDX_EMP_DEPT | 5 | | 0 (0)| 00:00:01 |
| 6 | TABLE ACCESS BY INDEX ROWID | EMP | 5 | 45 | 1 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T2"."LOC"='CHICAGO')
5 - access("T1"."DEPTNO"="T2"."DEPTNO")
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
7 consistent gets
0 physical reads
0 redo size
714 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
6 rows processed如果把hash 连接outline data加入hint，讲不会使用新建的索引，走出hash连接：
select t1.ename,t1.deptno from emp t1 where t1.deptno in (select t2.deptno from dept t2 where t2.loc='CHICAGO');
6 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2711458306
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5 |100 | 5 (0)| 00:00:01 |
|* 1 | HASH JOIN | | 5 |100 | 5 (0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 1 | 11 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | IDX_DEPT_LOC | 1 | | 1 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL | EMP | 14 |126 | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."DEPTNO"="T2"."DEPTNO")
3 - access("T2"."LOC"='CHICAGO')
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
9 consistent gets
0 physical reads
0 redo size
714 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
6 rows processed所有hint由_optimizer_ignore_hints决定（system或者session级别），默认false，不忽略hint，设置成ture将会忽略掉所有的hint。
SQL> alter system set "_optimizer_ignore_hints"=true;
System altered.
SQL> select * from emp where empno=7369;
Execution Plan
----------------------------------------------------------
Plan hash value: 2949544139
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 38 | 1 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| EMP | 1 | 38 | 1 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | PK_EMP | 1 | | 0 (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("EMPNO"=7369)
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
889 bytes sent via SQL*Net to client
512 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed免责声明：
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