通过实例认识MySQL中前缀索引的用法

root@test 07:57:52>alter table article drop index ind_article_url;
Query OK, 144384 rows affected (16.29 sec)
Records: 144384 Duplicates: 0 Warnings: 0
root@test 07:58:40>alter table article add index ind_article_url(url);
Query OK, 144384 rows affected, 1 warning (19.52 sec)
Records: 144384 Duplicates: 0 Warnings: 0
root@test 07:59:23>show warnings;
+———+——+———————————————————+
| Level  | Code | Message                         |
+———+——+———————————————————+
| Warning | 1071 | Specified key was too long; max key length is 767 bytes |
+———+——+———————————————————+
1 row in set (0.00 sec)

`URL` varchar(512) default NULL COMMENT ‘外链url',
……
KEY `ind_article_url` (`URL`(383))
…..
DEFAULT CHARSET=gbk
……
drop table test;
create table test(test varchar(767) primary key)charset=latin5;

drop table test;
create table test(test varchar(768) primary key)charset=latin5;

ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes
drop table test;
create table test(test varchar(383) primary key)charset=GBK;

drop table test;
create table test(test varchar(384) primary key)charset=GBK;

 ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes
drop table test;
create table test(test varchar(255) primary key)charset=UTF8;

drop table test;
create table test(test varchar(256) primary key)charset=UTF8;

ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

root@test 08:10:35>select count(distinct(url))/count() from article;
+——————————-+
| count(distinct(url))/count() |
+——————————-+
|            0.0750 |
+——————————-+

root@test 08:16:41>select count(distinct left(url,76))/count() url_76,
-> count(distinct left(url,77))/count() url_77,
-> count(distinct left(url,78))/count() url_78,
-> count(distinct left(url,79))/count() url_79,
-> count(distinct left(url,80))/count() url_80,
-> count(distinct left(url,81))/count() url_81,
-> count(distinct left(url,82))/count() url_82,
-> count(distinct left(url,83))/count() url_83,
-> count(distinct left(url,84))/count() url_84,
-> count(distinct left(url,85))/count() url_85
-> from article;
+——–+——–+——–+——–+——–+——–+——–+——–+——–+——–+
| url_76 | url_77 | url_78 | url_79 | url_80 | url_81 | url_82 | url_83 | url_84 | url_85 |
+——–+——–+——–+——–+——–+——–+——–+——–+——–+——–+
| 0.0747 | 0.0748 | 0.0749 | 0.0749 | 0.0749 | 0.0749 | 0.0749 | 0.0749 | 0.0749 | 0.0750 |
+——–+——–+——–+——–+——–+——–+——–+——–+——–+——–+
1 row in set (1.82 sec)

root@test 08:49:24>explain select id,url,deleted from article group by url;
+—-+————-+————-+——+—————+——+———+——+——–+———————————+
| id | select_type | table    | type | possible_keys | key | key_len | ref | rows  | Extra              |
+—-+————-+————-+——+—————+——+———+——+——–+———————————+
| 1 | SIMPLE   | article | ALL | NULL     | NULL | NULL  | NULL | 139844 | Using temporary; Using filesort |
+—-+————-+————-+——+—————+——+———+——+——–+———————————+
1 row in set (0.00 sec);