索引长度过长 ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

  今天在修改innodb表的某个列的长度时，报如下错误：

alter table test2 modify column id varchar(500);
ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

2.1 问题原因分析

其实从上面的报错中我们已经能看是因为索引达到长度限制，所以报错。查看id列是否有索引：

mysql> show index from test2G;
*************************** 1. row ***************************
        Table: test2
   Non_unique: 1
     Key_name: id
 Seq_in_index: 1
  Column_name: id
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment: 
Index_comment: 
1 row in set (0.00 sec)

2.2 关于mysql索引长度限制

1)单列索引长度限制

From the manual at http://dev.mysql.com/doc/refman/5.6/en/create-table.html >>从5.6的官方文档中我们能找到如下双引号中解释
"For CHAR, VARCHAR, BINARY, and VARBINARY columns, indexes can be created that use only the leading part of column values, using col_name(length) syntax to specify an index prefix length.
...
Prefixes can be up to 1000 bytes long (767 bytes for InnoDB tables). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE TABLE statements is interpreted as number of characters ...">>>对于myisam和innodb存储引擎，prefixes的长度限制分别为1000 bytes和767 bytes。注意prefix的单位是bytes，但是建表时我们指定的长度单位是字符。

A utf8 character can use up to 3 bytes. Hence you cannot index columns or prefixes of columns longer than 333 (MyISAM) or 255 (InnoDB) utf8 characters.  >>以utf8字符集为例，一个字符占3个bytes。因此在utf8字符集下，对myisam和innodb存储引擎创建索引的单列长度不能超过333个字符和255个字符

create table test2(id varchar(256),primary key(id));
ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

  从上面可以看出，mysql 在创建单列索引的时候对列的长度是有限制的 myisam和innodb存储引擎下长度限制分别为1000 bytes和767 bytes。(注意bytes和character的区别)

  对于innodb存储引擎，多列索引的长度限制如下：

每个列的长度不能大于767 bytes；所有组成索引列的长度和不能大于3072 bytes

参考如下例子(下面默认使用的使用innodb存储引擎，smallint 占2个bytes，timestamp占4个bytes，utf8字符集。utf8字符集下，一个character占三个byte)

mysql> create table test3(id varchar(255),key(id));
Query OK, 0 rows affected (0.11 sec)  >>索引列长度小于767(255*3=765),表创建成功

mysql> drop table test3;
Query OK, 0 rows affected (0.03 sec)

mysql> create table test3(id varchar(256),key(id));  >>索引列长度大于767(256*3=768),所以创建表失败
ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes
mysql> create table test3 (id varchar(255),name varchar(255),name1 varchar(255),name2 varchar(255),name3 varchar(5),key (id,name,name1,name2,name3));  >>innodb下多列索引，所有列长度和大于3072/3=1024 (255*4+5=1025),所以表创建失败
ERROR 1071 (42000): Specified key was too long; max key length is 3072 bytes
mysql> create table test3 (id varchar(255),name varchar(255),name1 varchar(255),name2 varchar(255),name3 varchar(4),key (id,name,name1,name2,name3));  >>
Query OK, 0 rows affected (0.09 sec)

  对于myisam存储引擎，多列索引长度限制如下：

每个列的长度不能大于1000 bytes，所有组成索引列的长度和不能大于1000 bytes

例子如下

mysql> create table test3(id varchar(334),key(id)) engine=myisam; >>索引列长度大于1000 bytes (334*3=1002)，建表报错
ERROR 1071 (42000): Specified key was too long; max key length is 1000 bytes
mysql> create table test3(id varchar(333),key(id)) engine=myisam;  >>索引列长度小于1000 bytes (333*3=999),建表成功
Query OK, 0 rows affected (0.07 sec)

mysql> create table test3(id varchar(300),name varchar(34),key(id,name)) engine=myisam; >>多列索引，所有列长度大于1000 bytes 建表报错
ERROR 1071 (42000): Specified key was too long; max key length is 1000 bytes
mysql> 

1) using col_name(length) syntax to specify an index prefix length.

对列的前面某部分创建索引

2) 启用innodb_large_prefix参数

innodb_large_prefix >>启用innodb_large_prefix参数能够取消对于索引中每列长度的限制(但是无法取消对于索引总长度的限制)
启用innodb_large_prefix有如下前提条件：
Enable this option to allow index key prefixes longer than 767 bytes (up to 3072 bytes) for InnoDB tables that use the DYNAMIC and COMPRESSED row formats.(Creating such tables also requires the option values innodb_file_format=barracuda and innodb_file_per_table=true.) >>启用innodb_large_prefix必须同时指定innodb_file_format=barracuda，innodb_file_per_table=true，并且建表的时候指定表的row_format为dynamic或者compressed(mysql 5.6中row_format默认值为compact)
See Section 14.6.7, “Limits on InnoDB Tables” for the relevant maximums associated with index key prefixes under various settings. >>
For tables using the REDUNDANT and COMPACT row formats, this option does not affect the allowed key prefix length.

例子如下：

1. 查看innodb_large_prefix，innodb_file_format参数
mysql> show variables like 'innodb_large_prefix';
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| innodb_large_prefix | OFF   |
+---------------------+-------+
1 row in set (0.01 sec)

mysql> show variables like 'innodb_file_format';
+--------------------+----------+
| Variable_name      | Value    |
+--------------------+----------+
| innodb_file_format | Antelope |
+--------------------+----------+
1 row in set (0.00 sec)

2. 建索引测试(innodb_large_prefix,innodb_file_format都为默认值的情况下)
mysql> create table test3(id varchar(256),key (id));  
ERROR 1709 (HY000): Index column size too large. The maximum column size is 767 bytes.

mysql> create table test3(id varchar(255),name varchar(255),name1 varchar(255),name2 varchar(255),name3 varchar(5),key (id,name,name1,name2,name3));
ERROR 1071 (42000): Specified key was too long; max key length is 3072 bytes
##索引列大于767 bytes时报错，组合索引列的总长度大于3072 bytes时报错

3. 修改innodb_large_prefix,innodb_file_format参数
mysql> set global innodb_large_prefix=1;
Query OK, 0 rows affected (0.00 sec)
mysql> set global innodb_file_format=BARRACUDA;
Query OK, 0 rows affected (0.00 sec)

4. 对row_format为dynamic格式表创建索引测试
mysql> create table test3(id varchar(256),key (id)) row_format=dynamic;
Query OK, 0 rows affected (0.14 sec)  

mysql> create table test3(id varchar(255),name varchar(255),name1 varchar(255),name2 varchar(255),name3 varchar(5),key (id,name,name1,name2,name3)) row_format=dynamic; 
ERROR 1071 (42000): Specified key was too long; max key length is 3072 bytes
##innodb_large_prefix=1并且innodb_file_format=BARRACUDA时，对于row_format为dynamic的表可以指定索引列长度大于767 bytes。但是索引列总长度的不能大于3072 bytes的限制仍然存在