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关于Oracle中执行计划稳定性深入研究

Jun 07, 2016 pm 03:01 PM
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欢迎进入Oracle社区论坛,与200万技术人员互动交流 >>进入 什么是执行计划 所谓执行计划,顾名思义,就是对一个查询任务,做出一份怎样去完成任务的详细方案。举个生活中的例子,我从珠海要去英国,我可以选择先去香港然后转机,也可以先去北京转机,或者去

欢迎进入Oracle社区论坛,与200万技术人员互动交流 >>进入

  什么是执行计划

    所谓执行计划,顾名思义,就是对一个查询任务,做出一份怎样去完成任务的详细方案。举个生活中的例子,我从珠海要去英国,我可以选择先去香港然后转机,也可以先去北京转机,或者去广州也可以。但是到底怎样去英国划算,也就是我的费用最少,这是一件值得考究的事情。同样对于查询而言,我们提交的SQL仅仅是描述出了我们的目的地是英国,但至于怎么去,通常我们的SQL中是没有给出提示信息的,是由数据库来决定的。

  我们先简单的看一个执行计划的对比:


SQL> set autotrace traceonly
  
  执行计划一:
  
  SQL> select count(*) from t;
  
  COUNT(*)
  ----------
  24815
  
  Execution Plan
  
    0   SELECT STATEMENT Optimizer=CHOOSE
  
  1  0  SORT (AGGREGATE)
  
  2  1   TABLE ACCESS (FULL) OF 'T'
  
  执行计划二:
  
  SQL> select count(*) from t;
  
  COUNT(*)
  
  24815
  
  Execution Plan
  
  0   SELECT STATEMENT Optimizer=CHOOSE (Cost=26 Card=1)
  
  1  0  SORT (AGGREGATE)
  
  2  1   INDEX (FULL SCAN) OF 'T_INDEX' (NON-UNIQUE) (Cost=26 Card=28180)
 

  
    这两个执行计划中,第一个表示求和是通过进行全表扫描来做的,把整个表中数据读入内存来逐条累加;第二个表示根据表中索引,把整个索引读进内存来逐条累加,而不用去读表中的数据。但是这两种方式到底哪种快呢?通常来说可能二比一快,但也不是绝对的。这是一个很简单的例子演示执行计划的差异。对于复杂的SQL(表连接、嵌套子查询等),执行计划可能几十种甚至上百种,但是到底那种最好呢?我们事前并不知道,数据库本身也不知道,但是数据库会根据一定的规则或者统计信息(statistics)去选择一个执行计划,通常来说选择的是比较优的,但也有选择失误的时候,这就是这次讨论的价值所在。

Oracle优化器模式

Oracle优化器有两大类,基于规则的和基于代价的,在SQLPLUS中我们可以查看init文件中定义的缺省的优化器模式。


  
  SQL> show parameters optimizer_mode
  
  NAME                 TYPE  VALUE
  
    
  optimizer_mode           string  CHOOSE
  
  SQL>
 

  
    这是Oracle8.1.7 企业版,我们可以看出,默认安装后数据库优化器模式为CHOOSE,我们还可以设置为 RULE、FIRST_ROWS,ALL_ROWS。可以在init文件中对整个instance的所有会话设置,也可以单独对某个会话设置:


  
  SQL> ALTER SESSION SET optimizer_mode = RULE;
  
  会话已更改。
  
  SQL> ALTER SESSION SET optimizer_mode = FIRST_ROWS;
  
  会话已更改。
  
  SQL> ALTER SESSION SET optimizer_mode = ALL_ROWS;
  
  会话已更改。

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关于Oracle中执行计划稳定性深入研究

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