关于cursor_sharing=similar

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关于cursor_sharing=similar

我们先看看在表没有分析无统计数据情况下的表现

SQL>[color=red] alter session set cursor_sharing = similar; [/color]

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
[color=red]parse count (hard)                                                     1619 (硬分析次数)[/color]
parse count (failures)                                                   80

SQL>[color=red] select count(*) from t where object_id = 1000; [/color]

   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
[color=red]parse count (hard)                                                     1620 [/color]
parse count (failures)                                                   80

SQL> [color=red]select count(*) from t where object_id = 1000; [/color]

   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
[color=red]parse count (hard)                                                     1620 [/color]
parse count (failures)                                                   80

SQL> [color=red]select count(*) from t where object_id = 1001; [/color]

   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
[color=red]parse count (hard)                                                     1620(即使object_id发生变化依然没有硬解析)[/color]
parse count (failures)                                                   80

我们再来看分析表和字段信息后的表现

SQL>[color=red] analyze table t1 compute statistics for table for columns object_id; [/color]

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
[color=red]parse count (hard)                                                     1640 [/color]
parse count (failures)                                                   80

SQL>[color=red] select count(*) from t1 where object_id = 5000; [/color]

   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
[color=red]parse count (hard)                                                     1641 [/color]
parse count (failures)                                                   80

SQL>[color=red] select count(*) from t1 where object_id = 5000; [/color]

   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
[color=red]parse count (hard)                                                     1641 (重复执行没发生变化)[/color]
parse count (failures)                                                   80

SQL>[color=red] select count(*) from t1 where object_id = 5001; [/color]

   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
[color=red]parse count (hard)                                                     1642 (当object_id变化的时候产生硬分析)[/color]
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。

原文参考
[url]http://blog.csdn.net/biti_rainy/archive/2004/07/12/learn_oracle_20040712_3.aspx[/url]
更多内容参考
[url]http://blog.csdn.net/biti_rainy[/url]