The diff in the pull request is one line: ALTER TABLE orders ALTER COLUMN total TYPE bigint;. It reads like a rounding fix, so the reviewer approves it, and the SQL is correct. On a table with 41 million rows it’s also a full rewrite under a lock that stops every read and write until it finishes. A diff shows what changes, never what the change costs.

A schema diff worth using would print consequences instead of SQL, in sentences like “this rewrites a 41-million-row table and blocks every read and write” or “the rollback is impossible without a backup”. Those sentences depend on three things: the statement, the engine and its version, and the size of the table it lands on. With all three you can post a risk report on every migration pull request. With only the first you have a linter.