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关于 cursor_sharing = similar

原创 Linux操作系统 作者:paulyibinyi 时间:2009-02-14 00:00:57 0 删除 编辑

ZT

http://hi.baidu.com/slash_gao/blog/item/c96db24332e558179313c611.html

我们先看看在表没有分析无统计数据情况下的表现

SQL> alter session set cursor_sharing = similar;

Session altered.

SQL> select name,value from v$sysstat where name like '%parse%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170148
parse count (hard)                                                     1619 (硬分析次数)
parse count (failures)                                                   80

SQL> select count(*) from t where object_id = 1000;

   COUNT(*)
----------
          0

SQL> select name,value from v$sysstat where name like '%parse%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170172
parse count (hard)                                                     1620
parse count (failures)                                                   80

SQL> /

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170176
parse count (hard)                                                     1620
parse count (failures)                                                   80

SQL> select count(*) from t where object_id = 1000;

   COUNT(*)
----------
          0

SQL> select name,value from v$sysstat where name like '%parse%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170178
parse count (hard)                                                     1620
parse count (failures)                                                   80

SQL> select count(*) from t where object_id = 1001;

   COUNT(*)
----------
          0

SQL> select name,value from v$sysstat where name like '%parse%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4948
parse time elapsed                                                     4468
parse count (total)                                                  170180
parse count (hard)                                                     1620(即使object_id发生变化依然没有硬解析)
parse count (failures)                                                   80

我们再来看分析表和字段信息后的表现

SQL> analyze table t1 compute statistics for table for columns object_id;

Table analyzed.

SQL> select name,value from v$sysstat where name like '%parse%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4973
parse time elapsed                                                     4495
parse count (total)                                                  170982
parse count (hard)                                                     1640
parse count (failures)                                                   80

SQL> select count(*) from t1 where object_id = 5000;


   COUNT(*)
----------
          0

SQL> select name,value from v$sysstat where name like '%parse%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4973
parse time elapsed                                                     4495
parse count (total)                                                  170984
parse count (hard)                                                     1641
parse count (failures)                                                   80

SQL> select count(*) from t1 where object_id = 5000;

   COUNT(*)
----------
          0

SQL> select name,value from v$sysstat where name like '%parse%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4973
parse time elapsed                                                     4495
parse count (total)                                                  171008
parse count (hard)                                                     1641 (重复执行没发生变化)
parse count (failures)                                                   80

SQL> select count(*) from t1 where object_id = 5001;

   COUNT(*)
----------
          0

SQL> select name,value from v$sysstat where name like '%parse%';

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
parse time cpu                                                         4973
parse time elapsed                                                     4495
parse count (total)                                                  171010
parse count (hard)                                                     1642 (当object_id变化的时候产生硬分析)
parse count (failures)                                                   80

SQL>

SQL> select sql_text,child_number from v$sql where sql_text like 'select count(*) from t1 where%';

SQL_TEXT
--------------------------------------------------------------------------------
CHILD_NUMBER
------------
select count(*) from t1 where object_id = :"SYS_B_0"
            0

select count(*) from t1 where object_id = :"SYS_B_0"
            1

        可以看出若存在object_id的 histograms ,则每次是不同的 值 的时候都产生硬解析 ,若不存在 histograms ,则不产生硬解析 。换句话说,当表的字段被分析过存在histograms的时候,similar 的表现和exact一样,当表的字段没被分析不存在histograms的时候,similar的表现和force一样。这样避免了一味地如force一样转换成变量形式,因为有hostograms的情况下转换成变量之后就容易产生错误的执行计划,没有利用上统计信息。而exact呢,在没有hostograms的情况下也要分别产生硬解析,这样的话,由于执行计划不会受到数据分布的影响(因为没有统计信息)重新解析是没有实质意义的。而similar则综合了两者的优点。

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/7199859/viewspace-551869/,如需转载,请注明出处,否则将追究法律责任。

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