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关于cursor_sharing = similar (zt)

原创 Linux操作系统 作者:tolywang 时间:2006-12-25 00:00:00 0 删除 编辑

摘要:本文通过简单实验来尝试说明cursor_sharing=similar的含义。


1.1. 实验现象

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

1.2. 结论

可以看出若存在object_id 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histograms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histograms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/35489/viewspace-84480/,如需转载,请注明出处,否则将追究法律责任。

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