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数据库select的默认排序

Jun 07, 2016 pm 03:47 PM
oracle ord select sort database default

oracle对无order by的语句返回的结果不进行排序,oracle此时的处理方式是按照数据的物理存储顺序来读取数据。因为rowid是每行数据的地址,所以有时候看起来会像是使用rowid排序的。但这个顺序是可能被打乱的,在表的数据被删除后,rowid会被新插入的数据占用

 oracle对无order by的语句返回的结果不进行排序,oracle此时的处理方式是按照数据的物理存储顺序来读取数据。因为rowid是每行数据的地址,所以有时候看起来会像是使用rowid排序的。但这个顺序是可能被打乱的,在表的数据被删除后,rowid会被新插入的数据占用。所以一个无order by查询结果看起来也可能是个杂乱无章的。

 

 Select 语句如果不加 “Order By”, MySQL会怎么排序呢?

* 不能依赖 MySQL 的默认排序
* 如果你想排序,总是加上 Order By
* GROUP BY 强加了 Order By,

对于 MyISAM 表
MySQL Select 默认排序是按照物理存储顺序显示的。(不进行额外排序).
也就是说
SELECT * FROM tbl – 会产生“表扫描”。如果表没有删除、替换、更新操作,记录会显示为插入的顺序。

InnoDB 表
同样的情况,会按主键的顺序排列。但是这是不靠谱的。潜规则。

“Select” 不加 “Order by”时, MySQL 会尝试以尽可能快的方法(MySQL 实际的方法不见得快)返回数据。
由于访问主键、索引大多数情况会快一些(在Cache里)所以返回的数据有可能以主键、索引的顺序输出,
这里并不会真的进行排序,主要是由于主键、索引本身就是排序放到内存的,所以连续输出时可能是某种序列。
在一些情况下消耗硬盘寻道时间最短的数据会先返回。
如果只查询单个表,在特殊的情况下是有规律的。

最后总结

“Order By 是要加的”

我们对于翻页等逻辑必须默认加上order by排序,而且order by的字段如果有重复值,必须指定第二排序字段,如果第二排序字段还有重复值,那必须指定更多的字段,直到所有的排序字段能够指定唯一顺序。

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