mysql中, varchar,blob/text等1页存储不下的时候, 行内就存储20字节关键信息, 真实数据就放到溢出页(overflow pages).

参考:
https://www.modb.pro/db/1789931525286465536

https://dev.mysql.com/doc/refman/8.0/en/innodb-file-space.html

https://dev.mysql.com/blog-archive/mysql-8-0-innodb-introduces-lob-index-for-faster-updates/

https://dev.mysql.com/blog-archive/externally-stored-fields-in-innodb/