(transfer) mysql: error 1071 (42000): specified key was too long; max key length is 767 bytes

Today's test encountered such a problem ：

 

mysql>  use test; create table test(a varchar(512) primary key, b varchar(1024));
Database changed
ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

 

Related tests ：

http://kklinux.com/bbs/archiver/tid-2543.html

 

drop table if exists test;
create table test(test varchar(767) primary key)charset=latin5;
-- success

drop table if exists test;
create table test(test varchar(768) primary key)charset=latin5;
-- error   
-- ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

drop table if exists test;
create table test(test varchar(383) primary key)charset=GBK;
-- success

drop table if exists test;
create table test(test varchar(384) primary key)charset=GBK;
-- error   
-- ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

drop table if exists test;
create table test(test varchar(255) primary key)charset=UTF8;
-- success

drop table if exists test;
create table test(test varchar(256) primary key)charset=UTF8;
-- error   
-- ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes

 

reason ：

MySQL Of varchar The primary key only supports no more than 768 Bytes perhaps 768/2=384 Two bytes perhaps 768/3=256 Three byte fields
and GBK It's double byte ,UTF-8 It's three bytes .

 

 

Variable is set ：

mysql> show variables like '%char%';
+--------------------------+--------------------------------------------------+
| Variable_name            | Value                                            |
+--------------------------+--------------------------------------------------+
| character_set_client     | latin1                                           |
| character_set_connection | latin1                                           |
| character_set_database   | utf8                                             |
| character_set_filesystem | binary                                           |
| character_set_results    | latin1                                           |
| character_set_server     | utf8                                             |
| character_set_system     | utf8                                             |
| character_sets_dir       | /opt/schooner/ac_3.1/mysql/share/mysql/charsets/ |
+--------------------------+--------------------------------------------------+
8 rows in set (0.00 sec)

 

So what are the rules of the character set ？ Find an article like this ：

http://www.blogjava.net/wldandan/archive/2007/09/04/142669.html

 

Some time ago , Has been mysql The character set of , Today, I would like to sum up the knowledge in this regard .
MySQL The character set of supports (Character Set Support) There are two aspects ：
     Character set (Character set) And sort them (Collation).
Support for character sets is refined to four levels :
     The server (server), database (database), Data sheet (table) And connections (connection).

1.MySQL Default character set

  MySQL The specification of character set can be refined to a database , A watch , A column of , What character set should be used .

however , Conventional The program does not use such complex configuration when creating databases and data tables , They use the default configuration , that , Where does the default configuration come from ？

    (1) compile MySQL when , Specifies a default character set , This character set is latin1;
    (2) install MySQL when , Can be found in the configuration file (my.ini) Specify a default character set in , If not specified , This value inherits from the value specified at compile time ;
    (3) start-up mysqld when , You can specify a default character set in the command line arguments , If not specified , This value is inherited from the configuration file , here character_set_server Set to the default character set ;
    (4) When creating a new database , Unless otherwise specified , The character set of this database is set to... By default character_set_server ;
    (5) When a database is selected ,character_set_database Set as the default character set for this database ;
    (6) When creating a table in this database , The default character set of the table is set to character_set_database , The default character set of this database ;
    (7) When setting a column in a table , Unless otherwise specified , Otherwise, the default character set of this column is the default character set of the table ;

Just to summarize , If nothing is changed , Then all fields of all tables in all databases are used latin1 Storage , But if we install MySQL, It's common to choose multilingual support , in other words , The setup program will automatically put... In the configuration file default_character_set Set to UTF-8, This ensures that by default , All fields of all tables in all databases use UTF-8 Storage .

2. View the default character set ( By default ,mysql The character set of is latin1(ISO_8859_1)
Usually , To view the character set and sorting mode of the system, you can use the following two commands ：
     mysql> SHOW VARIABLES LIKE 'character%';
+--------------------------+---------------------------------+
| Variable_name            | Value                           |
+--------------------------+---------------------------------+
| character_set_client     | latin1                          |
| character_set_connection | latin1                          |
| character_set_database   | latin1                          |
| character_set_filesystem | binary                          |
| character_set_results    | latin1                          |
| character_set_server     | latin1                          |
| character_set_system     | utf8                            |
| character_sets_dir       | D:"mysql-5.0.37"share"charsets" |
+--------------------------+---------------------------------+

  mysql> SHOW VARIABLES LIKE 'collation_%';
+----------------------+-----------------+
| Variable_name        | Value           |
+----------------------+-----------------+
| collation_connection | utf8_general_ci |
| collation_database   | utf8_general_ci |
| collation_server     | utf8_general_ci |
+----------------------+-----------------+

3. Modify the default character set
  (1) The easiest way to modify it , Is to modify mysql Of my.ini Character set key values in the file ,
Such as    default-character-set = utf8
      character_set_server =  utf8
   After modification , restart mysql Service for ,service mysql restart
   Use mysql> SHOW VARIABLES LIKE 'character%'; see , Found that the database code has been changed to utf8
+--------------------------+---------------------------------+
| Variable_name            | Value                           |
+--------------------------+---------------------------------+
| character_set_client     | utf8                            |
| character_set_connection | utf8                            |
| character_set_database   | utf8                            |
| character_set_filesystem | binary                          |
| character_set_results    | utf8                            |
| character_set_server     | utf8                            |
| character_set_system     | utf8                            |
| character_sets_dir       | D:"mysql-5.0.37"share"charsets" |
+--------------------------+---------------------------------+

   (2) There is also a way to modify the character set , Is the use of mysql The order of
     mysql> SET character_set_client = utf8 ;
     mysql> SET character_set_connection = utf8 ;
     mysql> SET character_set_database = utf8 ;
     mysql> SET character_set_results = utf8 ;
     mysql> SET character_set_server = utf8 ;

     mysql> SET collation_connection = utf8 ;
     mysql> SET collation_database = utf8 ;
     mysql> SET collation_server = utf8 ;


Generally, even if the default character set of the table is set to utf8 And through UTF-8 Send query by encoding , You will find that the code stored in the database is still garbled . That's the problem connection On connection layer . The solution is to execute the following sentence before sending the query ：

SET NAMES 'utf8';

It is equivalent to the following three sentences of instructions ：
SET character_set_client = utf8;
SET character_set_results = utf8;
SET character_set_connection = utf8;

summary :
therefore , What database version to use , Whether it's 3.x, still 4.0.x still 4.1.x, It doesn't really matter to us , There are two important things ：
  1) Set the database code correctly .MySQL4.0 The following versions of the character set always default ISO8859-1,MySQL4.1 When installing, you will be given the option of . If you are going to use UTF- 8, When you create a database, you must specify UTF-8( You can also change it after you create it ,4.1 The above versions can also specify the character set of the table separately )
  2) Set the database correctly connection code . After setting the database code , When connecting to the database , You should specify connection The coding , For example, use jdbc When the connection , Specify the connection as utf8 The way .