怎么理解并掌握mysql索引之前缀索引
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有时候需要很长的索引字符串,这样会使得索引变的很大而且很慢.通常可以索引开始的部分字符,这样可以大大节省空间提升索引效率,但这样也会降低索引的选择性.索引的选择性是指,不重复的索引值和数据表的记录总数的比值,范围从1#T到1之间.索引的选择性越高则查询效率越高,因为选择性高的索引可以让mysql在查找时过滤掉更多的行,唯一索引的选择性是1,这是最好的索引选择性,性能也是最好的.
一般情况下某个列前缀的选择性也是足够高的,足以满足查询性能.对于BLOB,TEXT或者很长的varchar类型的列,必须使用前缀索引,因为mysql不允许索引这些列的完整长度.
诀窍在于要选择足够长的前缀以保证较高的选择性,同时又不能太长.前缀长的选择性接近于索引整个列.换句话说,前缀的基数应该接近于完整列的基数.
为了决定前缀合适长度,需要找到最常见值的列表,然后和最常见的前缀列表进行比较.
如下构建一张表:
mysql> use sakila;Reading table information for completion of table and column namesYou can turn off this feature to get a quicker startup with -ADatabase changedmysql> create table city_demo(city varchar(50) not null);Query OK, 0 rows affected (0.15 sec)mysql> insert into city_demo(city) select city from city;Query OK, 600 rows affected (0.11 sec)Records: 600 Duplicates: 0 Warnings: 0mysql> insert into city_demo(city) select city from city_demo;Query OK, 600 rows affected (0.09 sec)Records: 600 Duplicates: 0 Warnings: 0mysql> update city_demo set city=(select city from city order by rand() limit 1);Query OK, 1196 rows affected (0.85 sec)Rows matched: 1200 Changed: 1196 Warnings: 0
有了数据集,数据分布不是真实分布,仅为演示.首先找到最常见的城市列表:
mysql> select count(*) as cnt,city from city_demo group by city order by cnt desc limit 10;+-----+-------------------------+| cnt | city |+-----+-------------------------+| 7 | Oshawa || 7 | Uijongbu || 7 | Ktahya || 6 | Haiphong || 6 | Berhampore (Baharampur) || 6 | Urawa || 6 | Mysore || 6 | Witten || 6 | Sunnyvale || 6 | Esfahan |+-----+-------------------------+10 rows in set (0.01 sec)
如上每个值都出现了6-7次,现在找出最频繁出现城市的前缀,先从前缀字母开始:
mysql> select count(*) as cnt,left(city,3) as pref from city_demo group by pref order by cnt desc limit 10;+-----+------+| cnt | pref |+-----+------+| 28 | San || 16 | Cha || 14 | Hal || 12 | al- || 11 | Bat || 11 | Shi || 10 | Val || 10 | Ben || 10 | Bra || 9 | Tar |+-----+------+10 rows in set (0.00 sec)
每个前缀出现的都比原来城市次数多,因此唯一前缀比唯一城市要少得多,然后增加前缀长度,直到这个前缀的选择性接近完整列的选着性,计算合适前缀长度的一个办法计算完整列的选择性,并使前缀的选择性趋于完整列的选择性.如下计算完整列的选择性:
mysql> select count(distinct city)/count(*) from city_demo;+-------------------------------+| count(distinct city)/count(*) |+-------------------------------+| 0.4300 |+-------------------------------+1 row in set (0.01 sec)
计算前缀选择性趋于或接近0.43这个值:
mysql> select count(distinct left(city,3))/count(*) from city_demo;+---------------------------------------+| count(distinct left(city,3))/count(*) |+---------------------------------------+| 0.3350 |+---------------------------------------+1 row in set (0.01 sec)mysql> select count(distinct left(city,4))/count(*) from city_demo;+---------------------------------------+| count(distinct left(city,4))/count(*) |+---------------------------------------+| 0.4058 |+---------------------------------------+1 row in set (0.00 sec)mysql> select count(distinct left(city,5))/count(*) from city_demo;+---------------------------------------+| count(distinct left(city,5))/count(*) |+---------------------------------------+| 0.4208 |+---------------------------------------+1 row in set (0.00 sec)mysql> select count(distinct left(city,6))/count(*) from city_demo;+---------------------------------------+| count(distinct left(city,6))/count(*) |+---------------------------------------+| 0.4267 |+---------------------------------------+
查询显示当前缀长度达到5的时候,再增加长度,选择性提升幅度已经不大.
只看平均选择性是不够的,也有列外情况,需要考虑最坏情况下的选择性,平均选择性会让你认为前缀长度为3或4的索引已经足够,但是如果数据分布很不均匀就会有陷阱.
上面示例如果找到合适前缀长度,下面示例如何创建前缀索引:
mysql> alter table city_demo add key(city(5));Query OK, 0 rows affected (0.34 sec)Records: 0 Duplicates: 0 Warnings: 0
到此，相信大家对"怎么理解并掌握mysql索引之前缀索引"有了更深的了解，不妨来实际操作一番吧！这里是网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
