我有这样一个表：
CREATE TABLE `products` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(512) NOT NULL,
`description` text,
PRIMARY KEY (`id`),
) ENGINE=InnoDB AUTO_INCREMENT=38 DEFAULT CHARSET=utf8;
和一个像这样的：
CREATE TABLE `product_variants` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`product_id` int(11) unsigned NOT NULL,
`product_code` varchar(255) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `product_code` (`product_code`),
KEY `product_variant_product_fk` (`product_id`),
CONSTRAINT `product_variant_product_fk` FOREIGN KEY (`product_id`) REFERENCES `products` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1037 DEFAULT CHARSET=utf8;
和SQL语句像这样
SELECT p.id AS id, p.name AS name, p.description AS description, pv.id AS product_variant_id, pv.product_code AS product_code
FROM products p
INNER JOIN product_variants pv ON pv.product_id = p.id
ORDER BY p.name ASC
LIMIT 300 OFFSET 0;
这要是我解释给我这样的：
+----+-------------+-------+------+----------------------------+----------------------------+---------+---------+--------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+----------------------------+----------------------------+---------+---------+--------+----------------+
| 1 | SIMPLE | p | ALL | PRIMARY | NULL | NULL | NULL | 993658 | Using filesort |
| 1 | SIMPLE | pv | ref | product_variant_product_fk | product_variant_product_fk | 4 | db.p.id | 1 | |
+----+-------------+-------+------+----------------------------+----------------------------+---------+---------+--------+----------------+
2 rows in set (0.00 sec)
对于一百万行，这是非常缓慢的。 我试着与products.name添加索引：
ALTER TABLE products ADD INDEX `product_name_idx` (name(512));
这给出了这样的：
mysql> show indexes from products;
+----------+------------+------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| products | 0 | PRIMARY | 1 | id | A | 993658 | NULL | NULL | | BTREE | | |
| products | 1 | product_manf_fk | 1 | manufacturer_id | A | 18 | NULL | NULL | YES | BTREE | | |
| products | 1 | product_name_idx | 1 | name | A | 201 | 255 | NULL | | BTREE | | |
+----------+------------+------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
我认为Sub_part列显示已在索引（以字节为单位），作为描述的前缀， 这个页面 。
当我重新解释查询，我得到：
+----+-------------+-------+------+----------------------------+----------------------------+---------+---------+--------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+----------------------------+----------------------------+---------+---------+--------+----------------+
| 1 | SIMPLE | p | ALL | PRIMARY | NULL | NULL | NULL | 993658 | Using filesort |
| 1 | SIMPLE | pv | ref | product_variant_product_fk | product_variant_product_fk | 4 | db.p.id | 1 | |
+----+-------------+-------+------+----------------------------+----------------------------+---------+---------+--------+----------------+
2 rows in set (0.00 sec)
它看起来像没有被使用的新指标。 由于在描述此页 ，索引将不会被使用，如果他们前缀索引排序。 事实上，如果我截断数据提供：
alter table products modify `name` varchar(255) not null;
该解释得到：
+----+-------------+-------+-------+----------------------------+----------------------------+---------+----------------------------------------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+----------------------------+----------------------------+---------+----------------------------------------------+------+-------+
| 1 | SIMPLE | p | index | PRIMARY | product_name_idx | 767 | NULL | 300 | |
| 1 | SIMPLE | pv | ref | product_variant_product_fk | product_variant_product_fk | 4 | oh_2c98c233_69fe_4f06_ad0d_fe6f85a5beac.p.id | 1 | |
+----+-------------+-------+-------+----------------------------+----------------------------+---------+----------------------------------------------+------+-------+
我觉得背上这件事。 然而，它说这个页面是InnoDB表最多可以有767个字节的索引。 如果长度是以字节为单位，为什么它拒绝有超过255多？ 如果它在字符，它是如何决定每个UTF-8字符的长度？ 难道仅仅是假设3？
此外，正在使用该版本的MySQL：
mysql> select version();
+------------+
| version() |
+------------+
| 5.5.27-log |
+------------+
1 row in set (0.00 sec)
