What I check before merging a migration

Schema changes are the riskiest thing most of us ship. This is the list I go through before approving one.

1. Does it lock a big table? Adding an index without CONCURRENTLY blocks writes.
2. Is it backwards compatible? Old code will run against the new schema for a few minutes during the deploy.
3. Can it be undone? Dropping a column is forever. Stop using it first, drop it a week later.
4. Has it run on a copy of production data? Timings on an empty database mean nothing.
5. Is the default value safe? A new non-null column needs one, and filling it can be slow.

None of this is clever. It is just easy to forget at five on a Friday.