Home Database Mysql Tutorial MySQL 通过索引优化含ORDER BY的语句_MySQL

MySQL 通过索引优化含ORDER BY的语句_MySQL

Jun 01, 2016 pm 01:20 PM
database

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关于建立索引的几个准则:

1、合理的建立索引能够加速数据读取效率,不合理的建立索引反而会拖慢数据库的响应速度。

2、索引越多,更新数据的速度越慢。

3、尽量在采用MyIsam作为引擎的时候使用索引(因为MySQL以BTree存储索引),而不是InnoDB。但MyISAM不支持Transcation。

4、当你的程序和数据库结构/SQL语句已经优化到无法优化的程度,而程序瓶颈并不能顺利解决,那就是应该考虑使用诸如memcached这样的分布式缓存系统的时候了。

5、习惯和强迫自己用EXPLAIN来分析你SQL语句的性能。

一个很容易犯的错误:

不要在选择的栏位上放置索引,这是无意义的。应该在条件选择的语句上合理的放置索引,比如where,order by。

例子:

SELECT id,title,content,cat_id FROM article WHERE cat_id = 1;

上面这个语句,你在id/title/content上放置索引是毫无意义的,对这个语句没有任何优化作用。但是如果你在外键cat_id上放置一个索引,那作用就相当大了。

几个常用ORDER BY语句的MySQL优化:

1、ORDER BY + LIMIT组合的索引优化。如果一个SQL语句形如:

SELECT [column1],[column2],.... FROM [TABLE] ORDER BY [sort] LIMIT [offset],[LIMIT];

这个SQL语句优化比较简单,在[sort]这个栏位上建立索引即可。

2、WHERE + ORDER BY + LIMIT组合的索引优化,形如:

SELECT [column1],[column2],.... FROM [TABLE] WHERE [columnX] = [value] ORDER BY [sort] LIMIT[offset],[LIMIT];

这个语句,如果你仍然采用第一个例子中建立索引的方法,虽然可以用到索引,但是效率不高。更高效的方法是建立一个联合索引(columnX,sort)

3、WHERE + IN + ORDER BY + LIMIT组合的索引优化,形如:

SELECT [column1],[column2],.... FROM [TABLE] WHERE [columnX] IN ([value1],[value2],...) ORDER BY[sort] LIMIT [offset],[LIMIT];

这个语句如果你采用第二个例子中建立索引的方法,会得不到预期的效果(仅在[sort]上是using index,WHERE那里是using where;using filesort),理由是这里对应columnX的值对应多个。

这个语句怎么优化呢?我暂时没有想到什么好的办法,看到网上有便宜提供的办法,那就是将这个语句用UNION分拆,然后建立第二个例子中的索引:

SELECT [column1],[column2],.... FROM [TABLE] WHERE [columnX]=[value1] ORDER BY [sort] LIMIT[offset],[LIMIT]
UNION
SELECT [column1],[column2],.... FROM [TABLE] WHERE [columnX]=[value2] ORDER BY [sort] LIMIT[offset],[LIMIT]
UNION
……

但经验证,这个方法根本行不通,效率反而更低,测试时对于大部分应用强制指定使用排序索引效果更好点

4、不要再WHERE和ORDER BY的栏位上应用表达式(函数),比如:

SELECT * FROM [table] ORDER BY YEAR(date) LIMIT 0,30;

5、WHERE+ORDER BY多个栏位+LIMIT,比如

SELECT * FROM [table] WHERE uid=1 ORDER x,y LIMIT 0,10;

对于这个语句,大家可能是加一个这样的索引(x,y,uid)。但实际上更好的效果是(uid,x,y)。这是由MySQL处理排序的机制造成的。

以上例子你在实际项目中应用的时候,不要忘记在添加索引后,用EXPLAIN看看效果。

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