开发中两个 varchar 1000 的字段进行关联，作为一个mysql索引小白，就直接去表里加一个正常的索引，这会报了一个错，说我索引长度过长。在网上搜了都是说要改mysql配置文件。偶然看到一个大佬发的文章，才知道mysql这个索引长度是这样一个用法。
mysql报的错：1071 - Specified key was too long; max key length is 3072 bytes
SELECT
ROUND(SUM(LENGTH(`school_unit_title`)<10)*100/COUNT(`school_unit_title`),2) AS pct_length_10,
ROUND(SUM(LENGTH(`school_unit_title`)<20)*100/COUNT(`school_unit_title`),2) AS pct_length_20,
ROUND(SUM(LENGTH(`school_unit_title`)<50)*100/COUNT(`school_unit_title`),2) AS pct_length_50,
ROUND(SUM(LENGTH(`school_unit_title`)<80)*100/COUNT(`school_unit_title`),2) AS pct_length_80,
ROUND(SUM(LENGTH(`school_unit_title`)<100)*100/COUNT(`school_unit_title`),2) AS pct_length_100
FROM `z_school_unit`;