> Is #3 really a problem? I certainly wouldn't expect schema changes to perform well, and increasing complexity just to help in this area would seem silly.
Not the parent, but obviously they refer to the online schema migration problem. Without that online schema migrations are basically impossible. Or you get handed the burden of implementing or finding a tool that does it for you.
And yes, pt-online-schema-change[1] is such a tool, but for MySQL. MariaDB has this feature built-in with their `ALTER ONLINE TABLE` [2].
So something similar for Postgres would be godsent.
[1] https://www.percona.com/doc/percona-toolkit/LATEST/pt-online...