SQL migrations to Postgres under load

How to update the attribute value for all records in a table? How to add a primary or unique key to a table? How to split a table in two? If your application may be unavailable for some time for migrations, then the answers to these questions are not difficult. But what if you need to migrate hot without stopping the database and without disturbing others to work with it? And if the tables are large (hundreds of gigabytes), there are several databases and several application servers?

Nikolay will try to give answers in the form of practical advice to these and other questions that arise during schema and data migrations in PostgreSQL.