Comment by weird-eye-issue

This largely depends on the kind of software system you are working on. These sorts of DDL migrations are common on the kinds of Rails apps I’ve worked on over my career. Size of the table, traffic patterns, who uses associated features, tolerance for small downtime windows, or orchestrating a multi-phase zero-downtime migration are all ways to justify these migrations, and is preferable to alternatives that would be comparably over-engineered for that app’s business and technical context.

> that already has data in production

exactly: already has data. It’s not the statement that’s unsafe, it’s the size of the table. That’s what all pattern matching migration checkers get wrong.

You might be releasing a new feature gradually and you realised your schema is slightly wrong and want to alter a column type. You’ve got some tiny volume of data in one production cluster. Is it safe?

A pseudo rule determining the safety for any arbitrary migration that causes a rewrite could be:

   smt.is_rewrite and tbl.size < 10MB

Yes: on your tiny new table

No: on your 10TB orders table

To accurately model migration safety you don’t really care about the statement: you care about the effects (locks, rewrites, additions, etc). That’s what is safe or unsafe.