关于 cursor_sharing = similar

作者:biti_rainy （出处）
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> 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。