有时候需要索引很长的字符列，这会让索引变得大且慢。通常可以索引开始的部分字符，这样可以大大节约索引空间，从而提高索引效率。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（也称为基数，cardinality)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（以便节约空间）。前缀应该足够长，以使得前缀索引的选择性接近于索引的整个列。换句话说，前缀的”基数“应该接近于完整的列的”基数“。
为了决定前缀的合适长度，需要找到最常见的值的列表，然后和最常见的前缀列表进行比较。下面的示例是mysql官方提供的示例数据库
下载地址如下：
http://downloads.mysql.com/docs/sakila-db.zip
在示例数据库sakila中并没有合适的例子，所以从表city中生成一个示例表，这样就有足够数据进行演示：
mysql> select database();
+------------+
| database() |
+------------+
| sakila |
+------------+
1 row in set (0.00 sec) mysql> create table city_demo (city varchar(50) not null);
Query OK, 0 rows affected (0.02 sec) mysql> insert into city_demo (city) select city from city;
Query OK, 600 rows affected (0.08 sec)
Records: 600 Duplicates: 0 Warnings: 0 mysql> insert into city_demo (city) select city from city_demo;
Query OK, 600 rows affected (0.07 sec)
Records: 600 Duplicates: 0 Warnings: 0 mysql> update city_demo set city = ( select city from city order by rand() limit 1);
Query OK, 1199 rows affected (0.95 sec)
Rows matched: 1200 Changed: 1199 Warnings: 0 mysql>
因为这里使用了rand（）函数，所以你的数据会与我的不同，当然那不影响聪明的你。
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 |
+-----+--------------+
| 8 | Garden Grove |
| 7 | Escobar |
| 7 | Emeishan |
| 6 | Amroha |
| 6 | Tegal |
| 6 | Lancaster |
| 6 | Jelets |
| 6 | Ambattur |
| 6 | Yingkou |
| 6 | Monclova |
+-----+--------------+
10 rows in set (0.01 sec) mysql>
注意到查询结果，上面每个值都出现了6-8次。现在查找到频繁出现的城市前缀。先从3个前缀字母开始，然后4个，5个，6个：
mysql> select count(*) as cnt,left(city,3) as pref from city_demo group by pref order by cnt desc limit 10;
+-----+------+
| cnt | pref |
+-----+------+
| 25 | San |
| 15 | Cha |
| 12 | Bat |
| 12 | Tan |
| 11 | al- |
| 11 | Gar |
| 11 | Yin |
| 10 | Kan |
| 10 | Sou |
| 10 | Bra |
+-----+------+
10 rows in set (0.00 sec) mysql> select count(*) as cnt,left(city,4) as pref from city_demo group by pref order by cnt desc limit 10;
+-----+------+
| cnt | pref |
+-----+------+
| 12 | San |
| 10 | Sout |
| 8 | Chan |
| 8 | Sant |
| 8 | Gard |
| 7 | Emei |
| 7 | Esco |
| 6 | Ying |
| 6 | Amro |
| 6 | Lanc |
+-----+------+
10 rows in set (0.01 sec) mysql> select count(*) as cnt,left(city,5) as pref from city_demo group by pref order by cnt desc limit 10;
+-----+-------+
| cnt | pref |
+-----+-------+
| 10 | South |
| 8 | Garde |
| 7 | Emeis |
| 7 | Escob |
| 6 | Amroh |
| 6 | Yingk |
| 6 | Moncl |
| 6 | Lanca |
| 6 | Jelet |
| 6 | Tegal |
+-----+-------+
10 rows in set (0.01 sec)
mysql> select count(*) as cnt,left(city,6) as pref from city_demo group by pref order by cnt desc limit 10;
+-----+--------+
| cnt | pref |
+-----+--------+
| 8 | Garden |
| 7 | Emeish |
| 7 | Escoba |
| 6 | Amroha |
| 6 | Yingko |
| 6 | Lancas |
| 6 | Jelets |
| 6 | Tegal |
| 6 | Monclo |
| 6 | Ambatt |
+-----+--------+
10 rows in set (0.00 sec) mysql>
通过上面改变不同前缀长度发现，当前缀长度为6时，这个前缀的选择性就接近完整咧的选择性了。甚至是一样的。
当然还有另外更方便的方法，那就是计算完整列的选择性，并使其前缀的选择性接近于完整列的选择性。下面显示如何计算完整列的选择性：
mysql> select count(distinct city) / count(*) from city_demo;
+---------------------------------+
| count(distinct city) / count(*) |
+---------------------------------+
| 0.4283 |
+---------------------------------+
1 row in set (0.05 sec) mysql>
可以在一个查询中针对不同前缀长度的选择性进行计算，这对于大表非常有用，下面给出如何在同一个查询中计算不同前缀长度的选择性：
mysql> select count(distinct left(city,3))/count(*) as sel3,
-> count(distinct left(city,4))/count(*) as sel4,
-> count(distinct left(city,5))/count(*) as sel5,
-> count(distinct left(city,6))/count(*) as sel6
-> from city_demo;
+--------+--------+--------+--------+
| sel3 | sel4 | sel5 | sel6 |
+--------+--------+--------+--------+
| 0.3367 | 0.4075 | 0.4208 | 0.4267 |
+--------+--------+--------+--------+
1 row in set (0.01 sec) mysql>
可以看见当索引前缀为6时的基数是0.4267，已经接近完整列选择性0.4283。
在上面的示例中，已经找到了合适的前缀长度，下面创建前缀索引：
mysql> alter table city_demo add key (city(6));
Query OK, 0 rows affected (0.19 sec)
Records: 0 Duplicates: 0 Warnings: 0 mysql>
mysql> explain select * from city_demo where city like 'Jinch%';
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | city_demo | range | city | city | 20 | NULL | 2 | Using where |
+----+-------------+-----------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec) mysql>
可以看见正确使用刚创建的索引。
前缀索引是一种能使索引更小，更快的有效办法，但另一方面也有其缺点：
mysql无法使用其前缀索引做ORDER BY和GROUP BY，也无法使用前缀索引做覆盖扫描。
参考资料
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