** 本文作者 ** ** : biti_rainy ( _ [email protected] _ ) **
** 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 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 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。
作者简介：
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网名 coolyl
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