我有一张这样的桌子：
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;
和这样的 SQL 语句
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;
如果我解释给我这个：
+----+-------------+-------+------+----------------------------+----------------------------+---------+---------+--------+----------------+
| 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，这非常慢。我尝试在 products.name 上添加索引：
ALTER TABLE products ADD INDEX `product_name_idx` (name(512));
这给出了这个：
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 列显示了索引中的前缀（以字节为单位），如本页所述。
当我重新解释查询时，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)
看起来新索引没有被使用。如 本页所述，如果索引是前缀索引，则不会使用索引进行排序。事实上，如果我截断数据：
alter table products modify `name` varchar(255) not null;
解释给出：
+----+-------------+-------+-------+----------------------------+----------------------------+---------+----------------------------------------------+------+-------+
| 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 | |
+----+-------------+-------+-------+----------------------------+----------------------------+---------+----------------------------------------------+------+-------+
我认为支持这一点。但是，它在此页面上说 InnoDB 表最多可以有 767 字节的索引。如果长度以字节为单位，为什么它拒绝超过 255？如果是字符，它如何决定每个 UTF-8 字符的长度？只是假设3？
另外，我正在使用这个版本的 MySQL：
mysql> select version();
+------------+
| version() |
+------------+
| 5.5.27-log |
+------------+
1 row in set (0.00 sec)