How to add a field to a high-loaded database in a production table with 100 million records?

Question

MySQL

How can I add a field to a high-loaded database in a production table with 100 million records in different databases without it failing?

Answer 1

I can answer about mysql.
in principle, the problem is solved by establishing a second table, switching the code to it, catching up the difference in the data and renaming the tables.
in principle, the processes are automated. for example, the pt-online-schema-change plugin.
but the correct answer is regulation. those works not in prime time.