Home Database Mysql Tutorial 通过分析SQL语句的执行计划优化SQL

通过分析SQL语句的执行计划优化SQL

Jun 07, 2016 pm 06:02 PM
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基于代价的优化器是很聪明的,在绝大多数情况下它会选择正确的优化器,减轻了DBA的负担。但有时它也聪明反被聪明误,选择了很差的执行计划,使某个语句的执行变得奇慢无比

如何干预执行计划 - - 使用hints提示

基于代价的优化器是很聪明的,在绝大多数情况下它会选择正确的优化器,减轻了DBA的负担。但有时它也聪明反被聪明误,选择了很差的执行计划,使某个语句的执行变得奇慢无比。此时就需要DBA进行人为的干预,告诉优化器使用我们指定的存取路径或连接类型生成执行计划,从而使语句高效的运行。例如,如果我们认为对于一个特定的语句,执行全表扫描要比执行索引扫描更有效,则我们就可以指示优化器使用全表扫描。在Oracle中,是通过为语句添加hints(提示)来实现干预优化器优化的目的。

hints是oracle提供的一种机制,用来告诉优化器按照我们的告诉它的方式生成执行计划。我们可以用hints来实现:

1. 使用的优化器的类型

2.基于代价的优化器的优化目标,是all_rows还是first_rows。

3. 表的访问路径,是全表扫描,还是索引扫描,还是直接利用rowid。

4. 表之间的连接类型

5. 表之间的连接顺序

6. 语句的并行程度

除了”RULE”提示外,一旦使用的别的提示,语句就会自动的改为使用CBO优化器,此时如果你的数据字典中没有统计数据,就会使用缺省的统计数据。所以建议大家如果使用CBO或HINTS提示,则最好对表和索引进行定期的分析。

如何使用hints:

Hints只应用在它们所在sql语句块(statement block,由select、update、delete关键字标识)上,对其它SQL语句或语句的其它部分没有影响。如:对于使用union操作的2个sql语句,如果只在一个sql语句上有hints,则该hints不会影响另一个sql语句。

我们可以使用注释(comment)来为一个语句添加hints,一个语句块只能有一个注释,而且注释只能放在SELECT, UPDATE, or DELETE关键字的后面

使用hints的语法:

<ccid_code>{DELETE|INSERT|SELECT|UPDATE} /*+ hint [text] [hint[text]]... */ or {DELETE|INSERT|SELECT|UPDATE} --+ hint [text] [hint[text]]...</ccid_code>
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注解:

1.DELETE、INSERT、SELECT和UPDATE是标识一个语句块开始的关键字,包含提示的注释只能出现在这些关键字的后面,否则提示无效。

2.“+”号表示该注释是一个hints,该加号必须立即跟在”/*”的后面,中间不能有空格。

3.hint是下面介绍的具体提示之一,如果包含多个提示,则每个提示之间需要用一个或多个空格隔开。

4.text 是其它说明hint的注释性文本

如果你没有正确的指定hints,Oracle将忽略该hints,并且不会给出任何错误。

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