In my previous blog post, I explained the internals of the sorted index build process. The blog ended with saying there is one disadvantage. Beginning in MySQL 5.6, many DDLs including ALTER TABLE ADD INDEX became ONLINE. Meaning, when the ALTER is in progress, there can be concurrent SELECTS and DMLs. See the MySQL documentation [&]

https://www.percona.com/blog/2019/06/27/innodb-alter-table-add-index-and-insert-performance/