MySQL版本5.6.35html
在一個長度爲512字符的字段上建立unique key報錯mysql
CREATE DATABASE dpcs_metadata DEFAULT CHARACTER SET utf8; select * from information_schema.SCHEMATA; +--------------+--------------------+----------------------------+------------------------+----------+ | CATALOG_NAME | SCHEMA_NAME | DEFAULT_CHARACTER_SET_NAME | DEFAULT_COLLATION_NAME | SQL_PATH | +--------------+--------------------+----------------------------+------------------------+----------+ | def | information_schema | utf8 | utf8_general_ci | NULL | | def | dpcs_metadata | utf8 | utf8_general_ci | NULL | | def | mysql | utf8 | utf8_bin | NULL | | def | performance_schema | utf8 | utf8_general_ci | NULL | | def | test | utf8 | utf8_bin | NULL | +--------------+--------------------+----------------------------+------------------------+----------+ 5 rows in set (0.00 sec) use dpcs_metadata; create table raw_log_meta_data( id bigint NOT NULL AUTO_INCREMENT, app_id varchar(64), user_id varchar(128), file_path varchar(512), device_id varchar(128), update_time DATETIME, PRIMARY KEY (id), UNIQUE KEY (user_id), UNIQUE KEY (file_path) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes
MySQL用1到2個額外字節記錄該字段的長度，當字段長度小於等於255時使用1個字節記錄字段長度，當長度大於255時使用2~4個字節記錄字段長度，字段file_path的長度爲512*3+2=1538個字節，超過系統默認767字節數限制sql
select * from information_schema.character_sets where character_set_name in('latin1', 'utf8', 'utf8mb4'); +--------------------+----------------------+----------------------+--------+ | CHARACTER_SET_NAME | DEFAULT_COLLATE_NAME | DESCRIPTION | MAXLEN | +--------------------+----------------------+----------------------+--------+ | latin1 | latin1_swedish_ci | cp1252 West European | 1 | | utf8 | utf8_general_ci | UTF-8 Unicode | 3 | | utf8mb4 | utf8mb4_general_ci | UTF-8 Unicode | 4 | +--------------------+----------------------+----------------------+--------+ 3 rows in set (0.00 sec)
根據上圖所示，採用utf8編碼的字段最大長度爲255個字符時，255*3+1=756，是小於最大767限制的，能夠建立成功，以下app
create table raw_log_meta_data( id bigint NOT NULL AUTO_INCREMENT, app_id varchar(64), user_id varchar(128), file_path varchar(256), device_id varchar(128), update_time DATETIME, PRIMARY KEY (id), UNIQUE KEY (user_id), UNIQUE KEY (file_path) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; ERROR 1071 (42000): Specified key was too long; max key length is 767 bytes create table raw_log_meta_data( id bigint NOT NULL AUTO_INCREMENT, app_id varchar(64), user_id varchar(128), file_path varchar(255), device_id varchar(128), update_time DATETIME, PRIMARY KEY (id), UNIQUE KEY (user_id), UNIQUE KEY (file_path) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; Query OK, 0 rows affected (0.19 sec) desc raw_log_meta_data; +-------------+--------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------------+--------------+------+-----+---------+----------------+ | id | bigint(20) | NO | PRI | NULL | auto_increment | | app_id | varchar(64) | YES | | NULL | | | user_id | varchar(128) | YES | UNI | NULL | | | file_path | varchar(255) | YES | UNI | NULL | | | device_id | varchar(128) | YES | | NULL | | | update_time | datetime | YES | | NULL | | +-------------+--------------+------+-----+---------+----------------+ 6 rows in set (0.01 sec)
根據文檔所述ui
https://dev.mysql.com/doc/refman/5.6/en/create-index.htmlthis
Prefix support and lengths of prefixes (where supported) are storage engine dependent. For example, a prefix can be up to 767 bytes long for InnoDB tables or 3072 bytes if the innodb_large_prefix option is enabled. For MyISAM tables, the prefix length limit is 1000 bytes. 編碼
https://dev.mysql.com/doc/refman/5.6/en/innodb-parameters.html#sysvar_innodb_large_prefixurl
innodb_large_prefixspa
| Property | Value |
|---|---|
| Command-Line Format | --innodb-large-prefix[={OFF|ON}] |
| Introduced | 5.6.3 |
| System Variable | innodb_large_prefix |
| Scope | Global |
| Dynamic | Yes |
| Type | Boolean |
| Default Value | OFF |
Enable this option to allow index key prefixes longer than 767 bytes (up to 3072 bytes) for InnoDB tables that use DYNAMIC or COMPRESSED row format. (Creating such tables also requires the option values innodb_file_format=barracuda and innodb_file_per_table=true.) See Section 14.6.1.6, 「Limits on InnoDB Tables」 for maximums associated with index key prefixes under various settings..net
For tables that use REDUNDANT or COMPACT row format, this option does not affect the permitted index key prefix length.
https://dev.mysql.com/doc/refman/5.6/en/innodb-restrictions.html
By default, the index key prefix length limit is 767 bytes. See Section 13.1.13, 「CREATE INDEX Syntax」. For example, you might hit this limit with a column prefix index of more than 255 characters on a TEXT or VARCHAR column, assuming a utf8mb3 character set and the maximum of 3 bytes for each character. When theinnodb_large_prefix configuration option is enabled, the index key prefix length limit is raised to 3072 bytes for InnoDB tables that use DYNAMIC or COMPRESSED row format.
根據以上可知，若是須要在長度大於255字符的字段上建立索引，須要修改如下3個參數
1. innodb_file_format=barracuda
3. ROW_FORMAT=DYNAMIC or COMPRESSED
mysql> show variables like 'innodb_large_prefix'; +---------------------+-------+ | Variable_name | Value | +---------------------+-------+ | innodb_large_prefix | OFF | +---------------------+-------+ 1 row in set (0.00 sec) mysql> show variables like 'innodb_file_format'; +--------------------+----------+ | Variable_name | Value | +--------------------+----------+ | innodb_file_format | Antelope | +--------------------+----------+ 1 row in set (0.00 sec) mysql> show variables like 'innodb_file_per_table'; +-----------------------+-------+ | Variable_name | Value | +-----------------------+-------+ | innodb_file_per_table | ON | +-----------------------+-------+ 1 row in set (0.00 sec) mysql> set global innodb_large_prefix='on'; Query OK, 0 rows affected (0.00 sec) mysql> set global innodb_file_format='Barracuda'; Query OK, 0 rows affected (0.00 sec) mysql> show variables like 'innodb_large_prefix'; +---------------------+-------+ | Variable_name | Value | +---------------------+-------+ | innodb_large_prefix | ON | +---------------------+-------+ 1 row in set (0.00 sec) mysql> show variables like 'innodb_file_format'; +--------------------+-----------+ | Variable_name | Value | +--------------------+-----------+ | innodb_file_format | Barracuda | +--------------------+-----------+ 1 row in set (0.00 sec)
create table raw_log_meta_data( id bigint NOT NULL AUTO_INCREMENT, app_id varchar(64), user_id varchar(128), file_path varchar(512), device_id varchar(128), update_time DATETIME, PRIMARY KEY (id), UNIQUE KEY (user_id), UNIQUE KEY (file_path) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 ROW_FORMAT=DYNAMIC; Query OK, 0 rows affected (0.29 sec)