Its not essential to understand how MySQL® and Percona Server for MySQL build indexes. However, if you have an understanding of the processing, it could help when you want to reserve an appropriate amount of space for data inserts. From MySQL 5.7, developers changed the way they built secondary indexes for InnoDB, applying a bottom-up [&]

https://www.percona.com/blog/2019/05/08/mysql-innodb-sorted-index-builds/