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

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 --