MySQL fault case | error 1071 (42000): specified key was too long

MySQL The following error occurred in creating the table （5.7）

ERROR 1071 (42000): Specified key was too long; max key length is 3072 bytes

Check the official website ：

If innodb_large_prefix is enabled (the default), the index key prefix limit is 3072 bytes for InnoDB tables that use the DYNAMIC or COMPRESSED row format. If innodb_large_prefix is disabled, the index key prefix limit is 767 bytes for tables of any row format.

https://dev.mysql.com/doc/refman/5.7/en/innodb-limits.html

By default, the index of a single column cannot exceed 767 position , We can enable innodb_large_prefix=on Options , Extend constraint to 3072 byte.

# 1、 Open online 
set global innodb_large_prefix = 1;

# 2、 Make sure to turn on 
show variables like 'innodb_large_prefix';

# 3、 Configuration file configuration is enabled 
innodb_large_prefix=on

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in addition , If the length of the field added to the index is too long , Can be added instead Prefix index The way , as follows

#  Single column 
alter table t1 add index idx_c1(c1(10));
 
#  Multiple columns 
alter table t1 add index idx_c1_c2(c1(10),c2(10)); 

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Be careful , Adding prefix index length is not the longer the better , There is a question of selectivity ,

select 
    count(distinct  Name )/count(*)as a,
    COUNT(DISTINCT left( Name ,100)) as b, 
    COUNT(DISTINCT left( Name ,110)) as c 
from  Table name ;

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