v$sql中的executions以多长时间回计算的

如题，执行次数(executions)是从数据库启动后就开始计算的，还是只是最近几个小时的执行次数？能清除v$sql里面的信息重新来过吗？

SQL> create table test as select * from dba_objects;

表已创建。

SQL> select count(*) from test;

??COUNT(*)
----------
? ???50437

SQL> select EXECUTIONS
??2? ?from v$sql where sql_text='select count(*) from test';

EXECUTIONS
----------
? ?? ?? ?1

SQL> select count(*) from test;

??COUNT(*)
----------
? ???50437

SQL> select EXECUTIONS
??2? ?from v$sql where sql_text='select count(*) from test';

EXECUTIONS
----------
? ?? ?? ?2

SQL>??select count(*) from test;

??COUNT(*)
----------
? ???50437

SQL>??select EXECUTIONS
??2? ? from v$sql where sql_text='select count(*) from test';

EXECUTIONS
----------
? ?? ?? ?2

SQL> grant select on test to scott;　　//invalidation cursor

授权成功。

SQL> select EXECUTIONS
??2? ?from v$sql where sql_text='select count(*) from test';

未选定行

SQL> select count(*) from test;

??COUNT(*)
----------
? ???50437

SQL> select EXECUTIONS
??2? ?from v$sql where sql_text='select count(*) from test';

EXECUTIONS
----------
? ?? ?? ?1

一个小测试

SQL> alter system flush shared_pool;

系统已更改。

SQL> select count(*) from test;

??COUNT(*)
----------
? ???50437

SQL> /

??COUNT(*)
----------
? ???50437

SQL> alter session set optimizer_mode=rule;　//产生新version
会话已更改。

SQL> select count(*) from test;

??COUNT(*)
----------
? ???50437

SQL> /

??COUNT(*)
----------
? ???50437

SQL> alter session set optimizer_mode=choose;　//产生新version


会话已更改。

SQL> select count(*) from test;

??COUNT(*)
----------
? ???50437

SQL> select child_number,executions from v$sql where sql_text='select count(*) from test';

CHILD_NUMBER EXECUTIONS
------------ ----------
? ?? ?? ???0? ?? ?? ? 2
? ?? ?? ???1? ?? ?? ? 2
? ?? ?? ???2? ?? ?? ? 1

SQL> select sql_id from v$sql where sql_text='select count(*) from test';

SQL_ID
-------------
7b2twsn8vgfsc
7b2twsn8vgfsc
7b2twsn8vgfsc

SQL> select * from v$sql_shared_cursor where sql_id='7b2twsn8vgfsc';

SQL_ID? ?? ???ADDRESS??CHILD_AD CHILD_NUMBER U S O O S L S E B P I S T A B D L T
------------- -------- -------- ------------ - - - - - - - - - - - - - - - - - -
R I I R L I O S M U T N F A I T D L D B P C S R P T M B M R O P M F L
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
7b2twsn8vgfsc 1F5CA95C 226E0CB8? ?? ?? ?? ?0 N N N N N N N N N N N N N N N N N N
N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N

7b2twsn8vgfsc 1F5CA95C 226DEFA8? ?? ?? ?? ?1 N N N N N N N N N N N N N N N N N N
N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N Y N N N N

7b2twsn8vgfsc 1F5CA95C 226DABD8? ?? ?? ?? ?2 N N N N N N N N N N N N N N N N N N
N N N N N N N N N N N N N N N N N N N N N N N N N N N N N N Y N N N N??

查文档倒数第５个
OPTIMIZER_MODE_MISMATCH VARCHAR2(1) Parameter OPTIMIZER_MODE mismatch (for example, all_rows versus first_rows_1)