使用hint优化Oracle的运行计划 以及 SQL Tune Advisor的使用

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背景：

某表忽然出现查询很缓慢的情况。cost 100+ 秒以上；严重影响生产。


原SQL：

explain plan for 
select * from (
select ID id,RET_NO retNo, FROM_SYS fromSy, TO_SYS toSys, COMMAND_CODE commandCode, COMMAND, STATUS, 
EXT_CODE, ORIGN_CODE orignCode,error_message errorMessage, RE_F, RET_MSG retMsg 
from interface_table where ((command_code in(‘AASSS‘) 
			and  status in(‘F‘,‘E‘) and (re_f = ‘N‘) and FROM_SYS = ‘MEE‘)
			or (COMMAND_CODE in(‘XXXX‘,‘XXXX9‘) and FROM_SYS = ‘EXT‘ and RE_F = ‘N‘)
			) and MOD(id, 1) = 0  order by id) where rownum <= 100  ;
查看其运行计划：
SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY(‘PLAN_TABLE‘));
Plan hash value: 1871549687
?
----------------------------------------------------------------------------------------------------
| Id? | Operation???????????????????? | Name?????????????? | Rows? | Bytes | Cost (%CPU)| Time???? |
----------------------------------------------------------------------------------------------------
|?? 0 | SELECT STATEMENT????????????? |??????????????????? |??? 99 |?? 382K|?? 637?? (1)| 00:00:08 |
|*? 1 |? COUNT STOPKEY??????????????? |??????????????????? |?????? |?????? |??????????? |????????? |
|?? 2 |?? VIEW??????????????????????? |??????????????????? |?? 100 |?? 386K|?? 637?? (1)| 00:00:08 |
|*? 3 |??? TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE??? |?? 355 | 55735 |?? 637?? (1)| 00:00:08 |
|*? 4 |???? INDEX FULL SCAN?????????? | PK_INTERFACE_TABLE |? 1439 |?????? |?? 280?? (2)| 00:00:04 |
----------------------------------------------------------------------------------------------------


优化后的SQL：

explain plan for 
select * from (
select /*+ index(INT_TABLE IX_INT_TABLE_2)*/ ID id,RET_NO retNo, FROM_SYS fromSy, TO_SYS toSys, COMMAND_CODE commandCode, COMMAND, STATUS, 
EXT_CODE, ORIGN_CODE orignCode,error_message errorMessage, RE_F, RET_MSG retMsg 
from interface_table where ((command_code in(‘AASSS‘) 
			and  status in(‘F‘,‘E‘) and (re_f = ‘N‘) and FROM_SYS = ‘MEE‘)
			or (COMMAND_CODE in(‘XXXX‘,‘XXXX9‘) and FROM_SYS = ‘EXT‘ and RE_F = ‘N‘)
			) and MOD(id, 1) = 0 order by id) where rownum <= 100  ;
查看其运行计划： 

SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY(‘PLAN_TABLE‘));
Plan hash value: 3625182869
?
--------------------------------------------------------------------------------------------------------
| Id? | Operation?????????????????????? | Name???????????????? | Rows? | Bytes | Cost (%CPU)| Time???? |
--------------------------------------------------------------------------------------------------------
|?? 0 | SELECT STATEMENT??????????????? |????????????????????? |??? 99 |?? 382K| 19105?? (1)| 00:03:50 |
|*? 1 |? COUNT STOPKEY????????????????? |????????????????????? |?????? |?????? |??????????? |????????? |
|?? 2 |?? VIEW????????????????????????? |????????????????????? |?? 356 |? 1376K| 19105?? (1)| 00:03:50 |
|*? 3 |??? SORT ORDER BY STOPKEY??????? |????????????????????? |?? 356 | 55892 | 19105?? (1)| 00:03:50 |
|?? 4 |???? CONCATENATION?????????????? |????????????????????? |?????? |?????? |??????????? |????????? |
|*? 5 |????? TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE????? |??? 69 | 10833 |? 9552?? (1)| 00:01:55 |
|*? 6 |?????? INDEX RANGE SCAN????????? | IX_INTERFACE_TABLE_2 | 77145 |?????? |??? 99?? (0)| 00:00:02 |
|*? 7 |????? TABLE ACCESS BY INDEX ROWID| INTERFACE_TABLE????? |?? 287 | 45059 |? 9552?? (1)| 00:01:55 |
|*? 8 |?????? INDEX RANGE SCAN????????? | IX_INTERFACE_TABLE_2 | 77145 |?????? |??? 99?? (0)| 00:00:02 |
--------------------------------------------------------------------------------------------------------


比較：

查看运行计划。原来是使用 full scan - 当数据量大时很慢。优化后oracle优先走range scan，hint 的 index 是未处理标识字段的索引，正常情况下这个数据集合相对较小--------所以能够达到优化目的。

详细情况详细分析，我们必需要看实际的表存的业务数据。分析其业务关系找到最小业务集合。后者要看懂运行计划，依据rows, bytes, cost, time 找到最优项目。这个分析顺序不能倒置。

问题：为何使用 rownum 后，oracle运行计划会走full scan？


转：怎样看懂运行计划：http://jadethao.iteye.com/blog/1613943


====? section2 ====

http://blog.chinaunix.net/uid-77311-id-3233190.html

环境:
OS:Red Hat Linux As 5
DB:10.2.0.4
?
Oracle通过STA给出某个SQL运行建议,以下通过一个測试检查Oracle给出的优化建议是否正确.
?
1.建表并生成測试数据
SQL> create table tb_test(id number not null,name varchar2(30));
Table created.
SQL> create index idx_tb_test on tb_test(id);
Index created.
SQL> declare
begin
? for i in 1 .. 100000 loop
??? insert into tb_test values (i, ‘test‘);
??? commit;
? end loop;
end;
/
?
2.分析表
begin
? dbms_stats.gather_table_stats(ownname => ‘SCOTT‘, tabname => ‘TB_TEST‘);
end;

3.编造一个运行计划不正确的SQL

select/*+ full(t)*/ count(1) from scott.tb_test t where t.id=1

我们知道在ID列上有索引,这个SQL走索引是正确的运行计划,但这里强制oracle走全表扫描,然后通过STA,看oracle给出的运行计划是否正确.

4.创建TUNING_TASK并运行

declare
? l_task_name varchar2(30);
? l_sql?????? clob;
begin
? l_sql?????? := ‘select/*+ full(t)*/ count(1) from scott.tb_test t where t.id=1‘;
? l_task_name := DBMS_SQLTUNE.CREATE_TUNING_TASK(sql_text??? => l_sql,
???????????????????????????????????????????????? user_name?? => ‘SCOTT‘,
???????????????????????????????????????????????? scope?????? => ‘COMPREHENSIVE‘,
???????????????????????????????????????????????? time_limit? => 60,
???????????????????????????????????????????????? task_name?? => ‘task_name01‘,
???????????????????????????????????????????????? description => null);
dbms_sqltune.Execute_tuning_task(task_name => ‘task_name01‘);
end;

5.查看oracle给出的优化建议

SQL> set serveroutput on;
SQL> set long 999999999;
SQL> SELECT DBMS_SQLTUNE.REPORT_TUNING_TASK(‘task_name01‘) FROM DUAL;

DBMS_SQLTUNE.REPORT_TUNING_TASK(‘TASK_NAME01‘)
--------------------------------------------------------------------------------
GENERAL INFORMATION SECTION
-------------------------------------------------------------------------------
Tuning Task Name????????????????? : task_name01
Tuning Task Owner???????????????? : SYS
Scope???????????????????????????? : COMPREHENSIVE
Time Limit(seconds)?????????????? : 60
Completion Status???????????????? : COMPLETED
Started at??????????????????????? : 06/03/2012 00:07:58
Completed at????????????????????? : 06/03/2012 00:07:59
Number of SQL Profile Findings??? : 1


DBMS_SQLTUNE.REPORT_TUNING_TASK(‘MY_TASK_NAME01‘)
--------------------------------------------------------------------------------
-------------------------------------------------------------------------------
Schema Name: SCOTT
SQL ID???? : ga3q5tqjgsj5u
SQL Text?? : select/*+ full(t)*/ count(1) from scott.tb_test t where t.id=1

-------------------------------------------------------------------------------
FINDINGS SECTION (1 finding)
-------------------------------------------------------------------------------

1- SQL Profile Finding (see explain plans section below)
--------------------------------------------------------

DBMS_SQLTUNE.REPORT_TUNING_TASK(‘MY_TASK_NAME01‘)
--------------------------------------------------------------------------------
? A potentially better execution plan was found for this statement.

? Recommendation (estimated benefit: 84.11%)
? ------------------------------------------
? - Consider accepting the recommended SQL profile.
??? execute dbms_sqltune.accept_sql_profile(task_name => ‘task_name01‘,
??????????? replace => TRUE);

-------------------------------------------------------------------------------
EXPLAIN PLANS SECTION
-------------------------------------------------------------------------------

DBMS_SQLTUNE.REPORT_TUNING_TASK(‘TASK_NAME01‘)
--------------------------------------------------------------------------------

1- Original With Adjusted Cost
------------------------------
Plan hash value: 1372292586

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

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

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

-- 当前的运行计划
DBMS_SQLTUNE.REPORT_TUNING_TASK(‘TASK_NAME01‘)
--------------------------------------------------------------------------------
|?? 0 | SELECT STATEMENT?? |?????????? |???? 1 |???? 4 |???? 6?? (0)| 00:00:01 |

|?? 1 |? SORT AGGREGATE??? |?????????? |???? 1 |???? 4 |??????????? |????????? |

|*? 2 |?? TABLE ACCESS FULL| TB_TEST? ?|???? 1 |???? 4 |???? 6?? (0)| 00:00:01 |

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


Predicate Information (identified by operation id):
---------------------------------------------------

DBMS_SQLTUNE.REPORT_TUNING_TASK(‘TASK_NAME01‘)
--------------------------------------------------------------------------------

?? 2 - filter("T"."ID"=1)

?

-- Oracle给出的运行计划

2- Using SQL Profile
--------------------
Plan hash value: 847665939

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

DBMS_SQLTUNE.REPORT_TUNING_TASK(‘TASK_NAME01‘)
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
---
|?? 0 | SELECT STATEMENT? |?????????????? |???? 1 |???? 4 |???? 1?? (0)| 00:00:0
1 |
|?? 1 |? SORT AGGREGATE?? |?????????????? |???? 1 |???? 4 |??????????? |
? |
|*? 2 |?? INDEX RANGE SCAN| IDX_TB_TEST? ?|???? 1 |???? 4 |???? 1?? (0)| 00:00:0
1 |
--------------------------------------------------------------------------------
---


DBMS_SQLTUNE.REPORT_TUNING_TASK(‘TASK_NAME01‘)
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

?? 2 - access("T"."ID"=1)

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

从上面的输出能够看出Oracle给出的运行计划是正确的.能够使用例如以下方法使用正确的运行计划

begin
? dbms_sqltune.accept_sql_profile(task_name => ‘task_name01‘, replace => TRUE);
end;

-- The End --




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