Oracle SQL语句的运行环境分为多个不同的层次，主要包括实例级别，会话级别，语句级别，其优先级依次递增。即语句级别的执行环境具
有最高的优先权，会话级别次之，实例级别最低。反过来，实例级别的环境设置影响全局，而会话级别的则影响当前会话，语句级别的设置当然
也就只影响当前语句。由此可知，运行环境中每一个环节的参数都对最终的数据库性能或所执行的SQL语句有直接的影响。因此在对数据库优化
或调试SQL时，获得当前SQL语句运行环境显得尤为重要。为此，Oracle提供了三个重要的视图来获取不同级别的参数信息。
一、优化器性能视图名字

--下面的三个性能视图分别对应到不同的级别

scott@ORCL> select table_name from dict where table_name like 'V$%OPTIMIZER_ENV';

TABLE_NAME

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

V$SYS_OPTIMIZER_ENV

V$SES_OPTIMIZER_ENV

V$SQL_OPTIMIZER_ENV

--注：上述的3个视图实际上是Oracle 10053事件中参数的一个子集 

二、实例级别执行环境(视图V$SYS_OPTIMIZER_ENV)

--提供实例级别的环境执行的信息。Oracle 优化器在为SQL语句生成执行计划时，会 据实例级别参数的值来确定，如 optimizer_mode 用于

--确定优化器的模式，optimizer_index_cost_adj用于确定全表扫描与索引扫描之间成本开销的比值等。

--实例级别的参数可以使用下面的方式来设置

alter system set parameter=value scope=memory | both |spfile;

--我们来看看当前实例级别的运行环境

scott@ORCL> select

  2  name,

  3  value,

  4  isdefault

  5  from

  6  v$sys_optimizer_env

  7  order by 3,1;

NAME                               |VALUE          |ISD

-----------------------------------|---------------|---

statistics_level                   |all            |NO     -->非缺省值在此显示为NO

active_instance_count              |1              |YES

bitmap_merge_area_size             |1048576        |YES

cpu_count                          |1              |YES

cursor_sharing                     |exact          |YES

hash_area_size                     |131072         |YES

optimizer_dynamic_sampling         |2              |YES

optimizer_features_enable          |10.2.0.1       |YES

optimizer_index_caching            |0              |YES

optimizer_index_cost_adj           |100            |YES

optimizer_mode                     |all_rows       |YES

optimizer_secure_view_merging      |true           |YES

parallel_ddl_mode                  |enabled        |YES

parallel_dml_mode                  |disabled       |YES

parallel_execution_enabled         |true           |YES

parallel_query_mode                |enabled        |YES

parallel_threads_per_cpu           |2              |YES

pga_aggregate_target               |59392 KB       |YES

query_rewrite_enabled              |true           |YES

query_rewrite_integrity            |enforced       |YES

skip_unusable_indexes              |true           |YES

sort_area_retained_size            |0              |YES

sort_area_size                     |65536          |YES

star_transformation_enabled        |false          |YES

workarea_size_policy               |auto           |YES

--从上面的查询结果可知，当前系统中仅有statistics_level参数使用了非缺省值，statistics_level参数的缺省值为typical，而此处为all。

--对于实例级别任意参数的修改其影响都是全局的， 此应当慎重。当然，如果某个参数设置的不合理，其影响也是全局的， 此对于实例级别

--参数的合理设置，可以避免成百上千枯燥的SQL语句调整，反之整个数据库性能急剧下降也可能是 为实例级别参数设置不当所致。 

三、会话级别执行环境(视图V$SES_OPTIMIZER_ENV)

--与实例级别相对应的会话级别视图是V$SES_OPTIMIZER_ENV。此时session 级别的参数值会优先于实例级别的参数值。

--我们可以通过V$SYS_OPTIMIZER_ENV与V$SES_OPTIMIZER_ENV来获得当前或某个特定session的所有session级别参数值。

scott@ORCL> show parameter cursor_shar

NAME                                |TYPE       |VALUE

------------------------------------|-----------|------------------

cursor_sharing                      |string     |EXACT

scott@ORCL> alter session set cursor_sharing=force;

Session altered.

scott@ORCL> show parameter cursor_shar

NAME                                |TYPE       |VALUE

------------------------------------|-----------|------------------

cursor_sharing                      |string     |FORCE

scott@ORCL> alter session set optimizer_mode=first_rows_100;

Session altered.

scott@ORCL> show parameter optimizer_mode

NAME                                |TYPE                            |VALUE

------------------------------------|--------------------------------|--------------------

optimizer_mode                      |string                          |FIRST_ROWS_100 

scott@ORCL> alter session set db_file_multiblock_read_count=128;

Session altered.

scott@ORCL> select username,sid,serial# from v$session where username='SCOTT';

USERNAME                      |       SID|   SERIAL#

------------------------------|----------|----------

SCOTT                         |       159|         5

scott@ORCL> select name,value,isdefault

  2  from v$ses_optimizer_env

  3  where sid=&input_sid 

  4  minus

  5  select name,value,isdefault

  6  from v$sys_optimizer_env;

Enter value for input_sid: 159

old   3: where sid=&input_sid

new   3: where sid=159

NAME                               |VALUE          |ISD

-----------------------------------|---------------|---

_db_file_optimizer_read_count      |128            |NO

cursor_sharing                     |force          |NO

optimizer_mode                     |first_rows_100 |NO 

四、语句级别执行环境(视图V$SQL_OPTIMIZER_ENV)

--影响SQL语句运行的执行环境受到实例级别以及会话级别之外，最后的，优先级最高的级别即是语句级别。 此语句级别的设置直接影响最终

--SQL语句执行计划的产生。SQL语句级别的参数设置是通过提示来完成的。 

--在下面的例子中，我们通过为同一SQL 语句使用提示来使其运行在不同的环境之中。

scott@ORCL> select  ename,job,sal from emp where empno=7369;  -->此SQL语句会 据实例与会话级别的优化器环境

                                                                            -->的设置来生成SQL执行计划并执行

ENAME     |JOB      |       SAL                                             -->此时的no_hint不是一个有效的提示，被当成注释

----------|---------|----------

Henry     |CLERK    |      1400

scott@ORCL> select  ename,job,sal from emp where empno=7369;  -->此时该SQL语句使用了all_rows提示

                                                                             -->注，此语句与上一语句尽管得到的相同的结果

ENAME     |JOB      |       SAL                                              -->但运行在不同的优化器环境模式之中

----------|---------|----------                                              -->两者会有不同的sql_id

Henry     |CLERK    |      1400 

scott@ORCL> show parameter optimizer_index_cost_adj                          -->查看参数设置

NAME                                |TYPE       |VALUE

------------------------------------|-----------|------------------------------

optimizer_index_cost_adj            |integer    |100

scott@ORCL> show parameter cursor_sharing                                    -->查看参数设置

NAME                                |TYPE       |VALUE

------------------------------------|-----------|------------------------------

cursor_sharing                      |string     |EXACT

scott@ORCL> select avg(sal) from emp;                                        -->聚合运算

  AVG(SAL)

----------

      2335

scott@ORCL> alter session set cursor_sharing=force;                          -->基于session级别修改参数

Session altered.

scott@ORCL> alter session set optimizer_index_cost_adj=80;                   -->基于session级别修改参数

Session altered.

scott@ORCL> select avg(sal) from emp;                                        -->再次聚合运算

  AVG(SAL)

----------

      2335

-->下面的查询得到同一SQL语句由于不同运行环境的sql_id,child_number等   

scott@ORCL> select sql_id, child_number, sql_text,optimizer_env_hash_value oehv,child_address

  2  from v$sql where sql_text like '%avg(sal)%' and sql_text not like '%from v$sql%';

SQL_ID       |CHILD_NUMBER|SQL_TEXT                           |      OEHV|CHILD_AD

-------------|------------|-----------------------------------|----------|--------

5du23va3p3ad0|           0|select avg(sal) from emp           |2196913643|3015F434

5du23va3p3ad0|           1|select avg(sal) from emp           |3293423667|2CD868A0

-->获得同一SQL语句父游 与子游 执行环境不同的地方

scott@ORCL> select se0.name,se0.value as value_child_0,se1.value as value_child_1

  2  from v$sql_optimizer_env se0

  3  inner join v$sql_optimizer_env se1

  4  on se0.sql_id=se1.sql_id

  5  where se0.sql_id='&input_sql_id'

  6  and se0.child_number=0

  7  and se1.child_number=1

  8  and se0.name=se1.name

  9  and se0.value<>se1.value;

Enter value for input_sql_id: 5du23va3p3ad0

old   5: where se0.sql_id='&input_sql_id'

new   5: where se0.sql_id='5du23va3p3ad0'

NAME                                    |VALUE_CHILD_0            |VALUE_CHILD_1

----------------------------------------|-------------------------|-------------------------

cursor_sharing                          |exact                    |force

optimizer_index_cost_adj                |100                      |80

-->获得上一SQL语句运行时优化器的详细信息

-->我们看到父游 与子游 的信息全部被列出来

scott@ORCL> select address, name, value, isdefault,child_number

  2  from v$sql_optimizer_env

  3  where sql_id = '5du23va3p3ad0'

  4  order by child_number, isdefault, name;

ADDRESS |NAME                                    |VALUE                    |ISD|CHILD_NUMBER

--------|----------------------------------------|-------------------------|---|------------

30177094|sqlstat_enabled                         |true                     |NO |           0

30177094|statistics_level                        |all                      |NO |           0

30177094|active_instance_count                   |1                        |YES|           0

30177094|bitmap_merge_area_size                  |1048576                  |YES|           0

30177094|cpu_count                               |1                        |YES|           0

30177094|cursor_sharing                          |exact                    |YES|           0

30177094|hash_area_size                          |131072                   |YES|           0

30177094|optimizer_dynamic_sampling              |2                        |YES|           0

30177094|optimizer_features_enable               |10.2.0.1                 |YES|           0

30177094|optimizer_index_caching                 |0                        |YES|           0

30177094|optimizer_index_cost_adj                |100                      |YES|           0

30177094|optimizer_mode                          |all_rows                 |YES|           0

30177094|optimizer_secure_view_merging           |true                     |YES|           0

30177094|parallel_ddl_mode                       |enabled                  |YES|           0

30177094|parallel_dml_mode                       |disabled                 |YES|           0

30177094|parallel_execution_enabled              |true                     |YES|           0

30177094|parallel_query_mode                     |enabled                  |YES|           0

30177094|parallel_threads_per_cpu                |2                        |YES|           0

30177094|pga_aggregate_target                    |59392 KB                 |YES|           0

30177094|query_rewrite_enabled                   |true                     |YES|           0

30177094|query_rewrite_integrity                 |enforced                 |YES|           0

30177094|skip_unusable_indexes                   |true                     |YES|           0

30177094|sort_area_retained_size                 |0                        |YES|           0

30177094|sort_area_size                          |65536                    |YES|           0

30177094|star_transformation_enabled             |false                    |YES|           0

30177094|workarea_size_policy                    |auto                     |YES|           0

30177094|cursor_sharing                          |force                    |NO |           1

30177094|optimizer_index_cost_adj                |80                       |NO |           1

30177094|sqlstat_enabled                         |true                     |NO |           1

30177094|statistics_level                        |all                      |NO |           1

30177094|active_instance_count                   |1                        |YES|           1

30177094|bitmap_merge_area_size                  |1048576                  |YES|           1

30177094|cpu_count                               |1                        |YES|           1

30177094|hash_area_size                          |131072                   |YES|           1

30177094|optimizer_dynamic_sampling              |2                        |YES|           1

30177094|optimizer_features_enable               |10.2.0.1                 |YES|           1

30177094|optimizer_index_caching                 |0                        |YES|           1

30177094|optimizer_mode                          |all_rows                 |YES|           1

30177094|optimizer_secure_view_merging           |true                     |YES|           1

30177094|parallel_ddl_mode                       |enabled                  |YES|           1

30177094|parallel_dml_mode                       |disabled                 |YES|           1

30177094|parallel_execution_enabled              |true                     |YES|           1

30177094|parallel_query_mode                     |enabled                  |YES|           1

30177094|parallel_threads_per_cpu                |2                        |YES|           1

30177094|pga_aggregate_target                    |59392 KB                 |YES|           1

30177094|query_rewrite_enabled                   |true                     |YES|           1

30177094|query_rewrite_integrity                 |enforced                 |YES|           1

30177094|skip_unusable_indexes                   |true                     |YES|           1

30177094|sort_area_retained_size                 |0                        |YES|           1

30177094|sort_area_size                          |65536                    |YES|           1

30177094|star_transformation_enabled             |false                    |YES|           1

30177094|workarea_size_policy                    |auto                     |YES|           1

52 rows selected.

-->Author : Robinson Cheng  

-->Blog :   http://dong2008hong.blog.163.com  

五、总结

1、通过三个重要的优化器性能视图v$sys_optimizer_env,v$ses_optimizer_env,v$sql_optimizer_env可以获得SQL语句的优化器相关的信息

2、三个性能视图获得的数据实际上Oracle 10053 事件的一部分，即也可以通过Oracle 10053 事件进行剖析来得到相同的结果

3、为调试SQL语句设置不同级别的优化器相关参数提供了便利